Before he was fired by President Trump, former FBI director James B. Comey fielded practice questions in advance of meetings and wrote highly detailed notes afterward in his car. (Jason Aldag,Bastien Inzaurralde/The Washington Post)

In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

President Trump publicly denied for the first time that he had tried to pressure FBI Director James B. Comey to drop the investigation into former national security adviser Michael Flynn in January.

“No. No,” Trump told reporters during a news conference. “Next question.”

Comey’s associates revealed that he felt so apprehensive about his meetings with Trump that he brainstormed questions and answers with a small group of confidants. One associate said Comey wrote down as much as he could once he got back to his car. Trump fired Comey last week.

The news developments were part of a week of rolling bombshell stories in national security, including:

On Wednesday, the Justice Department picked a special counsel to lead the Russia investigation.

FBI Director Robert Mueller in 2013. (Saul Loeb/AFP via Getty Images)

It was the moment Democrats had been waiting for: The Justice Department appointed former FBI director Robert S. Mueller III as a special counsel to oversee the Russia investigation. Usually the attorney general would make the pick, but since Jeff Sessions had to recuse himself from the investigation, the task fell to Rod J. Rosenstein, the deputy attorney general.

This was only the second time in the nation’s history that a special counsel has been appointed like this.

Lawmakers from both parties welcomed the appointment, describing Mueller as nonpolitical, fiercely independent and fair. President Trump, however, described the investigation as “the single greatest witch hunt of a politician in American history!”

Mueller, who will oversee the FBI probe into Russian interference in the 2016 elections, has the power to bring criminal charges but can also ultimately close the investigation with no action.

Also on Wednesday, The Post reported that the House majority leader told his Republican colleagues last year that he thinks Putin “pays” Trump.

House Speaker Paul D. Ryan leaves the podium after his weekly news conference on Capitol Hill, May 18. (Melina Mara/The Washington Post)

During a private conversation in June last year, House Majority Leader Kevin McCarthy (R-Calif.) said: “There’s two people I think Putin pays: Rohrabacher and Trump.” Rep. Dana Rohrabacker (R-Calif.) has a long history of supporting Russia. House Speaker Paul D. Ryan (R-Wis.) cut the conversation short and told them not to leak the remarks.

A spokesman for Ryan said the exchange never happened. But after The Washington Post said it would cite a recording of the conversation in the story, the spokesman said the exchange was “an attempt at humor.”

The conversation provides an inside view of what GOP leaders were thinking of Trump’s relationship to Russia before he was confirmed as the party’s nominee — and it also shows how they wanted to keep it secret.

During the exchange, Ryan was adamant that the conversation would stay “off the record.”

“No leaks, all right?” Ryan said, adding: “This is how we know we’re a real family here.”

On Tuesday, The Post and others reported that Comey kept detailed notes of his conversations with President Trump.

FBI Director James B. Comey appears before the Senate Judiciary Committee, May 3, in Washington. (Matt McClain/The Washington Post)

Notes taken by Comey reveal that Trump pressured him to drop the investigation into Flynn, according to associates of the fired FBI chief. Trump pulled Comey aside after a national security meeting and urged him to “let this go,” according to individuals who have seen the memo. The story was first reported by the New York Times.

Lawmakers want to see the notes and other records at the FBI that pertain to Comey’s contacts with Trump. Rep. Jason Chaffetz (R-Utah), chairman of the House Oversight Committee, tweeted that he has his “subpoena pen ready.”

Trump hinted that he might have “tapes” of his meetings with Comey. But White House officials have refused to say whether the president’s conversations have been recorded.

Legal analysts agree this is the strongest evidence to date that Trump might have obstructed justice and that there could be enough evidence for a criminal case. But as one legal expert notes: “It all depends on what he said and how he said it.” Intent is a key element in building an obstruction case. Investigators have to find evidence that a person “corruptly” tried to influence the investigation and that was their intention.

Essentially: “You need somebody else that Trump may have spoken to that provides evidence of intent” — a very high bar.

On Monday, The Post reported that Trump discussed classified information with Russian officials.

During a May 10 meeting with Russia's Foreign Minister Sergei Lavrov and Ambassador to the U.S. Sergey Kislyak, Trump began describing details about an Islamic State terror threat, according to current and former U.S. officials. (The Washington Post)

The Washington Post reported that President Trump discussed highly classified information with the Russian foreign minister and the Russian ambassador during a meeting at the White House.

Trump discussed the “great intel” and revealed intelligence on an Islamic State threat related to the use of laptop computers on aircraft that was provided by a key partner — reportedly Israel. The material was so sensitive that it was not shared with other allies, and Israel had not given permission for the intelligence to be provided to Russia.

The disclosure jeopardized an important intelligence-sharing arrangement and hindered the ability to detect future threats.

Russian Ambassador Sergey Kislyak, who attended the Oval Office meeting, has been a major source of controversy for the Trump administration — first with Flynn, whose contacts with the Russian ambassador led to his resignation — and Attorney General Jeff Sessions, who had to recuse himself after it turned out he misled Congress about his own contacts with the Moscow diplomat.

Meanwhile, the FBI is still looking for a new director.

The FBI is still leading the investigation, but the director role remains empty. Interviews are underway, and the White House had indicated that Trump would make his decision before leaving on his first trip abroad Friday.

The top candidates are acting FBI director Andrew McCabe, former senator Joe Lieberman (I-Conn.), former Oklahoma governor Frank A. Keating (R) and former FBI official Richard A. McFeely.