Hotel staffers and Malaysian officials declined to say whether Najib and the other officials stayed overnight at the hotel, among the most expensive in Washington, or if they did stay, for how long.
White House press secretary Sarah Huckabee Sanders dismissed a question about the delegation's stay. "We certainly don't book their hotel accommodations," she told reporters Tuesday.
But signs of the Malaysia delegation's presence were obvious at the property. At lunchtime Monday, more than a dozen members of Najib's entourage relaxed in a lounge area reserved for hotel guests. That evening, they came and went from the hotel, sometimes returning to the valet stand with shopping bags.
Najib departed just before 8 p.m. Monday in his motorcade. He had a scheduled dinner with a business group. He returned to the hotel and rode up the escalator at nearly 10:30 p.m.
On Tuesday morning, dozens of delegation members convened in meeting rooms with name cards bearing the Malaysian coat of arms. Some attended a white-tablecloth breakfast in the hotel's Lincoln Library meeting room.
Events of this scale would probably mean hundreds of thousands of dollars in revenue for the Trump Organization, based on confirmed spending totals of other groups that have set up camp there. The company declined to comment.
Trump has come under fire for declining to divest of his interest in the hotel, which is now managed by his sons, creating opportunities for foreign governments and special interests to enrich the president while also seeking changes to U.S. policy, in Najib's case within a few minutes' time.
Lawsuits from government watchdog groups and inquiries from Democratic leaders and governments agencies, including the Government Accountability Office, are targeting the hotel, which operates in a building leased to his company by the federal government, which Trump oversees.
Ethics experts, citing the Constitution's emoluments clause, consider foreign government bookings at the hotel unacceptable. Though it has hosted hotel events for many foreign entities, as well as lobbying-group events and Republican fundraisers, the Trump Organization says it does not seek foreign governments as clients, keeps a separate log of their payments and plans to donate profits it receives from them to the U.S. treasury at the end of the year.
Najib was scheduled to return to Malaysia on Wednesday evening.
Trump's invitation to Najib also has drawn scathing criticism because the United States is conducting a probe — its largest kleptocracy investigation ever — into whether the prime minister diverted more than $1 billion from a Malaysian-government investment fund to his own bank accounts.
The Justice Department is aggressively investigating potential fraud surrounding the fund, known as 1MDB, and announced in June that prosecutors had filed forfeiture complaints seeking $540 million in assets.
U.S. prosecutors a year earlier had filed similar complaints seeking more than $1 billion in assets that they alleged were ill-gotten gains from an effort by Malaysian officials and their associates to misappropriate money from the government-owned fund. That means the total value of assets sought stands at nearly $1.7 billion, the Justice Department has said.
Among the things U.S. officials have sought to seize are New York City penthouses, Hollywood and Beverly Hills mansions, a private jet and even some future proceeds from the movies "The Wolf of Wall Street" and "Dumb and Dumber To." The 2016 complaint alleged that more than $730 million of what seemed to be 1MDB money was ultimately routed to the personal bank account of "Malaysian Official 1," a thinly veiled reference to Najib.
Najib has said on social media that "no crime was committed," and an investigation by the Malaysian attorney general determined that the money in Najib's account was a personal donation from the Saudi royal family. In 2015, President Barack Obama visited Najib in Malaysia to curry support for the Trans-Pacific Partnership.
The Malaysian delegation visiting this week included six other high-ranking officials, according to the Malaysian Embassy, among them the minister of foreign affairs, the minister of trade, the chief secretary, the ambassador to the United States and the director of the nation's security council.
Members of Najib's entourage repeatedly waved off a reporter's questions Monday evening and Tuesday morning. Inquiries to the Malaysian Embassy and the White House were not immediately returned.
Once at the White House, Najib was greeted by Trump, Vice President Pence and Secretary of State Rex Tillerson.
"It's great to have the prime minister of Malaysia and his very distinguished delegation with us today," Trump said, sitting across a conference table from Najib. Trump praised the prime minister for his stance on terrorism and for investing in U.S. businesses.
"Mr. Prime Minister, it's a great honor to have you in the United States and in the White House," he said.
Najib responded by saying he wanted to help "strengthen the U.S. economy" by purchasing Boeing airplanes and engines from General Electric, among other initiatives.
Matt Zapotosky and Drew Harwell contributed to this report.