The probe into possible collusion between the Trump campaign and the Kremlin is entering a new and perilous stage as investigators interview key witnesses and expand their scrutiny of U.S. President Donald Trump and his aides.

Mr. Trump's son-in-law and senior adviser Jared Kushner is scheduled to testify behind closed doors before Congress this week, marking the first time that Mr. Kushner gives testimony to lawmakers. Mr. Kushner was a constant presence during Mr. Trump's political rise, playing key roles in the election campaign, the presidential transition and now the White House. Read the statement released ahead of his testimony here.

Mr. Trump is also facing pressure from Congress on another Russia-related matter. Legislators are planning to vote this week on a bill with bipartisan support that would enact strict limits on Mr. Trump's ability to ease sanctions on Russia, a rebuke to the President's stated goal of improving relations with Moscow.

Story continues below advertisement

As the Russia investigation intensifies, Mr. Trump continues to insist that there is no wrongdoing to uncover while musing about taking dramatic action in response to the probe. On Saturday, he wrote on Twitter that the president has the "complete power" to pardon people after a report emerged that he had discussed with aides his authority to grant clemency in the context of the Russia investigation.

Donald Trump Jr. was told in an email that the Russian government wanted to boost Donald Trump’s election bid last year, according to three people with knowledge of the email. This in part led to the meeting with Russian lawyer Natalia Veselnitskaya, which included Mr. Trump Jr., Jared Kushner and Paul Manafort, among others. HILARY SWIFT/The New York Times

On Sunday, Mr. Trump once again fumed on Twitter about what he called "a phony Russian Witch Hunt" and "an excuse for a lost election." He also targeted his fellow Republicans, saying it was "very sad" that they "do very little to protect their President."



In recent days, Mr. Trump has lashed out at key figures in the Russia inquiry. He complained about the actions of his Attorney-General, Jeff Sessions – who recused himself from any Russia inquiry – in such strong terms that observers considered it an invitation to resign. And he warned special counsel Robert Mueller, who is conducting the Justice Department's Russia probe, against examining the Trump family's business dealings. Mr. Trump also alleged Mr. Mueller had "many" conflicts, which he promised to reveal "at some point."

Mr. Kushner, who will testify in closed sessions of the Senate and House intelligence committees on Monday and Tuesday, was originally one of three Trump aides scheduled to testify this week. Paul Manafort, the former chairman of the Trump campaign, and Donald Trump Jr., the President's eldest son, were supposed to attend a public hearing on Wednesday, but their lawyers reached a deal that postpones their testimony. The two men will instead provide documents to the Senate judiciary committee and be interviewed in private ahead of testifying in public. It is not clear when those private interviews will take place.

Mr. Kushner, Mr. Manafort and Mr. Trump Jr. were all present at a controversial meeting at Trump Tower in June, 2016, with Russian lawyer Natalia Veselnitskaya. Mr. Trump's son had welcomed the meeting after being told it would yield "ultrasensitive" information about Hillary Clinton that was part of a Russian government effort to help Mr. Trump.



Jared Kushner is scheduled to testify behind closed doors before Congress this week, marking the first time that Mr. Kushner gives sworn testimony to lawmakers. Ruth Fremson/The New York Times

The upcoming testimony is "a minefield" for Mr. Trump's associates, said Ryan Goodman, a former counsel at the U.S. Department of Defence who teaches law at New York University. "Any discrepancies whatsoever between [Mr. Kushner's] telling of the story under oath versus Don Jr.'s telling of the story – and of course Manafort's – is going to be very significant." Making false statements in an ongoing government inquiry is a federal crime, Prof. Goodman noted, and has been the basis of successful prosecutions in past political scandals.



The meeting is far from the only item on the agenda of investigators. Mr. Kushner is also under scrutiny for his contacts with Russians during the presidential transition. In December, he met with the head of a Russian state-run bank that is the subject of U.S. sanctions. According to The Washington Post, Mr. Kushner also met with then-Russian ambassador Sergey Kislyak in December and discussed setting up a secret communications channel with the Kremlin.

Story continues below advertisement

Lawmakers are also likely to question Mr. Kushner about omissions in his initial application for a U.S. government security clearance. At first, Mr. Kushner did not list any contacts with foreign government officials in response to a question on the form. He amended the disclosure months later, adding at least 100 such contacts, according to The New York Times. Deliberately omitting information from an application for security clearance is technically a crime, but it is almost never prosecuted.

The questioning of Mr. Kushner comes amid reports that Mr. Mueller, the special counsel, is expanding his probe to include the business dealings of Mr. Trump and his associates. That expansion is fraught with risks for Mr. Trump's team, experts say, since it could uncover wrongdoing unrelated to the Russia investigation but which nevertheless could be used as leverage.

Paul Manafort, Mr. Trump’s former campaign chief, is already facing legal jeopardy over failure to register his lobbying efforts on behalf of a pro-Russian political party in Ukraine, a disclosure required under U.S. law. Win McNamee/Getty Images

Mr. Mueller has selected for his team a "very hard-hitting set of prosecutors" who are "known to flip witnesses," said Jonathan Turley, a law professor at George Washington University. "Any collateral criminal act can be used as a means to coerce co-operation."



Mr. Manafort , the former campaign chief,is already facing legal jeopardy over his failure to register his lobbying efforts on behalf of a pro-Russian political party in Ukraine, a disclosure required under U.S. law. Such a failure is technically a crime, but again, one that is rarely prosecuted.

More problematic for Mr. Manafort could be that Mr. Mueller is reportedly investigating him for possible money laundering in connection with a series of real estate transactions in New York. Mr. Manafort borrowed tens of millions of dollars to finance housing purchases in recent years, The Wall Street Journal reported, but he maintains that they were all ordinary transactions. Federal prosecutors sought Mr. Manafort's bank records earlier this year, the paper reported.

Editor's note: Jared Kushner was questioned by congressional investigators on Monday but did not testify under oath, as this story originally stated. Making false statements intentionally in a congressional proceeding is a crime, whether or not a person is under oath.