As Donald Trump surged in the Republican primary polls in the early months of 2016, his outsider campaign faced growing pressure to show that the former reality-TV star and noted provocateur was forming a coherent and credible world view.

So when Carter Page, an international businessman with an office near Trump Tower, volunteered his services, former officials recall, Trump aides were quick to make him feel welcome.

He had come with a referral from the son-in-law of Richard Nixon, New York state Republican Party Chairman Ed Cox, who had conveyed Page’s interest to the campaign, Cox said.

A top Trump adviser, Sam Clovis, then employed what campaign aides now acknowledge was their go-to vetting process — a quick Google search — to check out the newcomer. He seemed to have the right qualifications, according to former campaign officials — head of an energy investment firm, business degree from New York University, doctorate from the University of London.

Page was in. He joined a new Trump campaign national security advisory group, and in late March 2016, the candidate pointed to Page, among others, as evidence of a foreign policy team with gravitas.

But what the Google search had not shown was that Page had been on the FBI’s radar since at least 2013, when Russian officials allegedly tried to use him to get information about the energy business.

By the summer of 2016, Page, who had been recently named as a Trump adviser, was under surveillance by FBI agents who suspected that he may have been acting as an agent of the Kremlin.

As part of its broader investigation into potential collusion between the Trump campaign and the Russian government, the FBI continues to examine how Page joined the campaign and what conversations he may have had with Russian officials about the effort to interfere with the election — with or without the knowledge of Trump and his team — according to people familiar with the matter.

The Senate Intelligence Committee has also zeroed in on Page, asking him for records of all his contacts with Russians during the campaign, all financial interactions he had with Russia and all communications he had with Trump campaign staff.

The circumstances that led to Page’s easy access to the Trump campaign represent one of the main questions facing investigators: Were Trump’s connections to multiple Russia-friendly advisers mere coincidence, or evidence of a coordinated attempt to collude with a foreign government? Or were they the result of incompetent vetting that left a neophyte candidate vulnerable to influence from people with nefarious agendas?

Regardless of the answer, the campaign’s previously unreported procedures for vetting Page and other advisers are greatly complicating matters for Trump’s presidency. Along with Page, a number of other Trump associates are under growing scrutiny by congressional investigators and the FBI as they examine potential ties between the campaign and Moscow, including former national security adviser Michael Flynn, onetime campaign chairman Paul Manafort and informal Trump adviser Roger Stone.

This week, former CIA director John Brennan told the House Intelligence Committee that in 2016 he had seen intelligence revealing “contacts and interactions between Russian officials and U.S. persons involved in the Trump campaign that I was concerned about because of known Russian efforts to suborn such individuals.” He did not name the individuals but said that when he stepped down as CIA director on Jan. 20, he had “unresolved questions” about whether the Russians had been successful in getting campaign associates to work on their behalf “in a witting or unwitting fashion.”

Carter Page, a former foreign policy adviser for President-elect Donald Trump, speaks at a news conference at the RIA Novosti news agency in Moscow on Dec. 12. Page said he was meeting with businessmen and politicians during his visit. (Pavel Golovkin/AP)

Multiple people familiar with campaign operations, most of whom spoke on the condition of anonymity to discuss internal deliberations, said that Page and others were brought into the fold at a time of desperation for the Trump team. As Trump was starting to win primaries, he was under increasing pressure to show that he had a legitimate, presidential-caliber national security team. The problem he faced was that most mainstream national security experts wanted nothing to do with him.

“Everyone did their best, but there was not as much vetting as there could have been,” former campaign manager Corey Lewandowski said.

Another longtime campaign official put it this way: “Anyone who came to us with a pulse, a résumé and seemed legit would be welcomed.”

“We were not exactly making due diligence the highest priority,” another campaign veteran added.

A White House spokeswoman referred questions to Trump’s campaign. Michael Glassner, who currently serves as manager of Trump’s campaign committee, declined to comment.

Page and Trump aides have said that Page never met Trump, and Page left the campaign in August 2016. Page has denied working on behalf of the Russians and said questions about his Moscow ties are part of a political witch hunt designed by Democrats to discredit Trump.

Page has for months declined to answer questions about how he joined the Trump team and who invited him aboard, calling the matter “irrelevant” in an email exchange with The Washington Post.

In a letter to Deputy Attorney General Rod J. Rosenstein last week, Page wrote that he had been “an informal, unpaid member of one of [Trump’s] campaign committees.”

In his defense, Page in recent weeks has sent a series of meandering letters to investigators. He has quoted Maroon 5 lyrics, cited the writings of George Orwell and said he is being persecuted because of his Catholic faith.

In a letter sent Wednesday to the House Intelligence Committee, Page referred to himself as an “unpaid, informal member of the Make America Great Again movement,” a reference to Trump’s campaign slogan, and said he had been “illegitimately swept up into this investigation” based on “false evidence and propaganda.”

Page’s entry to the campaign came as Trump was starting to win Republican primaries and take commanding leads in GOP polls — but was also facing criticism for his lack of foreign policy advisers.

In early March 2016, more than 70 conservative national security experts signed an open letter opposing Trump’s candidacy, calling him “fundamentally dishonest.” Trump announced that then-Sen. Jeff Sessions (R-Ala.), the only senator supporting him, would chair a foreign policy panel for the campaign — but no other members of the panel were named publicly.

When the hosts of MSNBC’s “Morning Joe” pressed Trump on air in mid-March to name people with whom he spoke about foreign affairs, the candidate’s response only seemed to underscore his lack of serious advice.

“I’m speaking with myself, number one, because I have a very good brain,” he said.

As the campaign maneuvered behind the scenes to expand its ranks, Page had at least one built-in advantage when he joined the Trump campaign: geographic proximity.

He ran a company, Global Energy Capital LLC, with offices located a block from Trump Tower and connected by an atrium to the famed property.

Some campaign veterans speculated that Page may have stepped forward on his own, essentially walking into campaign headquarters and introducing himself without a referral — a frequent occurrence at Trump Tower in those early days.

Last week, Page rejected this version of events. “Wrong again,” he told The Post via text message. “At least one other primary person(s) involved.”

On Thursday, after Cox described his role, Page confirmed that the New York GOP chairman had connected him to the Trump campaign.

Cox said in an interview that Page, an acquaintance from business and political circles, had reached out to him in early 2016 expressing interest in joining the Trump campaign. Cox said he routinely connected potential volunteers with GOP campaigns. He described Page as “very informed and up to date on things.”

Several former officials recall that when Page first showed up at Trump Tower, Lewandowski introduced him to other campaign aides. Lewandowski said he could not remember the encounter, which was first reported by the Daily Caller, but also did not rule it out.

Clovis, who assembled and vetted the list of national security advisers that included Page, declined to comment. Now a top official at the Agriculture Department, Clovis had worked on Russia-related issues at the Pentagon in the 1980s and, as a candidate for U.S. Senate in Iowa in 2014, had questioned the effectiveness of sanctions imposed after Russia’s incursion into Ukraine.

Representatives for Flynn, Sessions and Jared Kushner, Trump’s senior adviser, declined to comment. A White House official said senior policy adviser Stephen Miller, also a key campaign staffer at the time, had no role in the formation of the foreign policy group.

A thorough vetting of Page might have revealed several red flags. Page had spent three years working in Moscow, for instance, and he held stock in the Russian company Gazprom, meaning that he could have a personal financial stake in the future of U.S.-imposed sanctions against Russia.

Page wrote in a September letter to then-FBI Director James B. Comey that he had sold his “de minimis equity investment” in the Russian company at a loss a month earlier.

Page had previously drawn the attention of the FBI after he had conversations in 2013 with a man posing as an executive with the New York branch of the Russian development bank Vneshecon­ombank. The man was later convicted of being a Russian spy, and FBI recordings included discussions among Russian operatives about their attempts to recruit Page. Page has said that he cooperated with the FBI and that the only crime related to the incident is that U.S. government officials appear to have recently revealed his role to the media.

By late March 2016, when Trump appeared before The Post’s editorial board, he was prepared to brag about his new foreign policy team.

“I can give you some of the names,” Trump said after Post Publisher Frederick J. Ryan Jr. asked about his advisers.

Second on the list of five read aloud by Trump: “Carter Page, PhD.”

Another unusual name on Trump’s list of foreign policy experts was a little-known figure named George Papadopoulos, whose inclusion may also have demonstrated the vulnerabilities that came with limited vetting.

“He’s an energy consultant,” Trump said. “Excellent guy.”

The news media soon reported that Papadopoulos seemed to have exaggerated elements of his résumé. And, touting his position as a Trump adviser, Papadopoulos began offering positive comments about Russian President Vladi­mir Putin to foreign audiences.

Papadopoulos did not respond to requests for comment. His name had surfaced four months earlier on a similar list of foreign policy advisers circulated by the presidential campaign of Ben Carson.

Barry Bennett, who served as Carson’s campaign manager and later as an adviser to Trump, recalled that he was surprised when Trump named Papadopoulos to his team.

“He was someone who worked for me at the Carson campaign for, like, 15 minutes. And somehow he was on the list,” Bennett said of his reaction to the Trump roster. “I was, like, how in the hell did that happen?”

Trump soon added a few names to the group and held a formal briefing with its members in a conference room at his not-yet-opened hotel in Washington. A photo of that session distributed by Trump on social media — “meeting with my national security team,” Trump wrote — showed the candidate at the head of the table and Sessions at the opposite end.

Papadopoulos could be seen at the table. Page was not there, and Trump officials have said that Trump never met Page during his five-month stint as an adviser.

People involved with the campaign recall that Page attended other meetings that the group held in Washington, including one attended by Sessions. He also submitted policy memos for the campaign’s review, a former campaign adviser said.

In June, Page stunned a group of foreign policy luminaries during a private meeting at Blair House with the visiting prime minister of India by going off-topic to declare that Putin was a stronger and more reliable leader than President Barack Obama, according to people who were in the room. Page also promised that U.S.-Russian relations would improve if Trump were elected. Page has denied this account, blaming it on his political enemies.

The next month, Page delivered a speech at a Russian university in which he was highly critical of U.S. policy. Page has said he met with no Russian government officials during the trip, except for briefly greeting a deputy prime minister who attended the event.

Over the summer, the FBI convinced a Foreign Intelligence Surveillance Court judge that there was probable cause to believe Page was acting as an agent of Russia, law enforcement and other U.S. officials told The Post last month.

Page’s name also appeared in a now-famous dossier, which quietly circulated among reporters and alleged that there were links between Trump associates and the Kremlin. The document asserted that Page met with top Russian officials to plot how to elect Trump.

The document, which was compiled by a former British spy employed by Trump’s political adversaries, became public after the election and was dismissed by Trump and his allies as “fake news.” Page vigorously rejects the allegation about him and said the FBI should spend its time investigating how the document came together instead of his activities.

Page used his Wednesday letter to the House Intelligence Committee to defend his contacts with the Russians. Presenting the letter as a “follow-up” to Brennan’s testimony, Page wrote that throughout “my interactions with the Russians in 2016, I consistently made it crystal clear that all of my benign statements and harmless actions in Moscow as well as elsewhere overseas were solely made as a scholar and a business person speaking only on behalf of myself. In other words, in no way connected to then-candidate Trump.”

Earlier this month, Page insisted in writing to the Senate Intelligence Committee that for all the attention he has received, he did not once meet Trump.

“In retrospect and with the 1984-inspired governance standards employed in 2016,” Page wrote, “I consider it fortunate that I never briefed Mr. Trump.”

Note: This story has been updated.

Devlin Barrett and Alice Crites contributed to this report.