Michael Cohen is quickly emerging as one of the most dangerous cooperators not only against President Donald Trump, but also his son, Donald Trump Jr.

Cohen's plea deal with the special counsel Robert Mueller raises fresh questions about whether Trump Jr. misled congressional investigators about a now-defunct proposal to build a Trump Tower in Moscow.

Rep. Adam Schiff, the ranking member on the House Intelligence Committee said Cohen's plea deal indicates that "other witnesses were also untruthful before our committee."

A knowledgeable congressional source told INSIDER Trump Jr. was one of the witnesses to whom Schiff was referring.

When the FBI raided Michael Cohen's property earlier this year and it surfaced that he was the target of a federal criminal investigation, Justice Department veterans warned that if he were to flip, he could be the most dangerous cooperating witness against President Donald Trump and his family.

This week, the special counsel Robert Mueller's office revealed a slice of just how much dirt Cohen may have on Trump in a charging document laying out how Cohen misled congressional investigators last year about the Trump Organization's effort to build a Trump Tower in Moscow during the 2016 election.

Cohen and Trump have known each other for years, and in addition to being Trump's personal lawyer, he was also his longtime fixer and served as the Trump Organization's lead attorney for a decade.

Prosecutors said Cohen, who pleaded guilty to one count of lying to Congress, misled lawmakers when he said negotiations for the Trump Tower Moscow deal ended in January 2016 and that he did not discuss it extensively with Trump Organization executives. They said Cohen did so "in hopes of limiting the ongoing Russia investigations."

These revelations don't just spell trouble for Trump. They've also cast the spotlight back on Donald Trump Jr., Trump's eldest son who is an executive vice president at the Trump Organization.

Donald Trump Jr. Chip Somodevilla/Getty Images

Trump Jr. misled the House Intelligence Committee, per source

When he testified before the Senate Judiciary Committee last September, Trump Jr. told the panel he "wasn't involved" in the Trump Tower Moscow project and was only "peripherally aware" of it in 2016.

He also said he was not aware Cohen had reached out to the Russian government about the deal and indicated that Cohen's work with the Russian-born businessman Felix Sater on the project ended in 2015.

But prosecutors wrote that Cohen "discussed the status and progress of the Moscow Project" with Trump "on more than the three occasions Cohen claimed" to the Senate Intelligence Committee and that "he briefed family members" of Trump within the Trump Organization about it.

They also said Cohen admitted to pursuing the deal with Sater as late as June 2016, after Trump became the presumptive Republican presidential nominee.

After Cohen's plea deal was announced, Rep. Adam Schiff, the ranking member on the House Intelligence Committee, said in a statement that Cohen's plea "highlights concern over another issue – that we believe other witnesses were also untruthful before our committee."

A knowledgeable congressional source told INSIDER that Trump Jr. was one of the witnesses to whom Schiff was referring.

Michael Cohen. Yana Paskova/Getty Images

Trump Jr. 'ought to be alarmed'

Elie Honig, a former federal prosecutor from the Southern District of New York, told INSIDER the president's son "ought to be alarmed" by Cohen's plea deal. "We know Trump Jr. was questioned on the same topics that Michael Cohen has now been convicted of lying about."

It's unclear exactly what Trump Jr. told the House Intelligence Committee about the scrapped deal, because his interview took place in a closed-door session.

"The decision of whether to charge Trump Jr. with perjury is really going to come down to a question of precision," Honig said. "How clear is it that he misstated the truth? Did he leave himself enough wiggle room, and is his version of facts squarely contradicted by documents, emails, or other evidence showing his level of involvement?"

Honig added the caveat that Trump Jr. likely did not have the same level of involvement in the project as Cohen did. But, he added, "it's clear he did have some involvement at some time. So the key question is going to be, how much was Trump Jr. involved in, and how does that square off with the specifics of his testimony?"

Cohen has been cooperating with investigators since August, when he pleaded guilty to tax evasion, bank fraud, and campaign-finance violations in a separate Manhattan US attorney's office investigation into his and Trump's financial dealings before the election. His most recent sit-down with Mueller was last week.

Robert Mueller. AP Photo/Andrew Harnik

'You don't get a pass because your last name is Trump'

Cohen's is the first guilty plea Mueller has secured that is directly connected to Trump's business dealings in Russia. And he is the first person Mueller has prosecuted for lying to Congress.

But DOJ veterans say that doesn't mean he'll be the last.

"There are certainly others at the Trump Organization that knew about this Russian overture," Jeffrey Cramer, a longtime former federal prosecutor in Chicago, told INSIDER. "If they knew about it and lied, either to Mueller or to Congress, there's no reason to think they'd be treated differently than Cohen. You don't get a pass because your last name is Trump."

If it emerges that Trump Jr. knew more than he was letting on about the deal and gave false testimony about it to congressional committees, "he or any other individual is now staring at the same fate Cohen is," Cramer said. "It brings home to those in the company who may have lied about the deal that their time could be coming sooner rather than later."

This isn't the first time Trump Jr.'s congressional testimony has come under the microscope.

Lawmakers also questioned the president's son about his involvement in a June 2016 meeting at Trump Tower between top campaign officials and two Russian lobbyists offering dirt on the Hillary Clinton campaign as "part of Russia and its government's support" for Trump's candidacy.

Trump and his lawyers have said he did not know about the meeting until after it was reported on last July. Trump Jr. told the Senate Judiciary Committee the same, saying he did not inform his father of the meeting prior to its occurrence.

But CNN, citing sources with knowledge of the matter, reported in July that Cohen claims he was one of several people who were present when Trump Jr. informed Trump of the offer before the meeting. Cohen reportedly says that Trump greenlit the meeting after hearing about it from his son.

CNN and NBC News reported that Cohen does not have physical evidence — like a tape recording — to back up his claim but was willing to testify about it under oath to Mueller.