Preet Bharara: Former Trump campaign adviser Papadopoulos 'almost certainly' has 'flipped' Former Trump adviser is likely cooperating with investigators, Bharara said.

 -- Former U.S. Attorney Preet Bharara said the charges against Donald Trump’s former campaign adviser, George Papadopoulos, are the most significant development so far in the Russia investigation because they suggest he has "almost certainly" flipped to cooperate with investigators.

Bharara told ABC News Chief Anchor George Stephanopoulos on "This Week" Sunday that the fact that Papadopoulos "almost certainly ... has flipped and [is] cooperating with the government to provide substantial assistance with respect to someone else higher up in the food chain means you're going to see more charges coming."

Papadopoulos pleaded guilty to making false statements and omitting information during an interview with the FBI on Jan. 27, 2017, according to a federal statement of the offense that was unsealed Monday by special counsel Robert Mueller's team.

The charges against him send a message that “the Mueller team takes very seriously being lied to ... Lying to the FBI is a form of obstruction" of justice, said Bharara, who was part of a panel discussion on "This Week" about the Russia probe.

The special counsel's team "clearly feel very seriously about [obstruction], and some people should be worried,” said the former U.S. attorney for the Southern District of New York.

Also on the "This Week" panel was Ken Starr, who as a federal independent counsel in the 1990s investigated President Bill Clinton including in the Monica Lewinsky scandal.

Starr agreed with Bharara that Mueller sent a message: “'Don’t lie to the FBI.'"

"I think that is the message that Bob Mueller chose to send on this particular day, that it’s not just about Paul Manafort, it’s about the integrity of the investigation. Just tell the truth,” Starr said.

But Starr and Bharara disagreed on whether President Trump has “crossed the line" against interfering with an investigation.

"He's crossed the line a number of times," Bharara said. "It is a terrible thing for a president in this country to tell his Justice Department who to investigate, who to prosecute, and who to keep their hands off of."

In contrast, Starr said, Trump is "just spouting off" in his comments about the Justice Department and the Russia probe.

"He's expressing this frustration, but it's not crossing the line into criminality," Starr said.