Mueller is turning up the heat on Paul Manafort - and it could spell trouble for Trump

• Special counsel Robert Mueller is drilling down on former Trump campaign chairman Paul Manafort as he investigates Russia's interference in the election.
• Mueller's focus on Manafort could be an attempt to "flip" him against Trump.
• Even if Manafort doesn't have incriminating evidence against Trump, he could still be a source of valuable information to Mueller from a counterintelligence standpoint.

Paul Manafort, the former chairman of President Donald Trump's campaign, is caught in the crosshairs of special counsel Robert Mueller's investigation into Russian interference in the 2016 election, and recent revelations indicate that Mueller will continue to drill down on his findings.

The special counsel's particular inquiry into Manafort not only confirms that he is a key focus in the Russia investigation, but also highlights the likelihood that Mueller is digging for criminal conduct in an effort to "flip" Manafort as a witness against Trump and other campaign associates.

On Monday, CNN reported that US investigators obtained a warrant to wiretap Manafort before and after the election. Some information gleaned from the surveillance prompted concerns that Manafort had encouraged Russians to "help with the campaign," according to CNN.

The news came on the heels of a New York Times report which found that after obtaining a "no-knock" warrant and conducting a predawn raid on Manafort's home in July, investigators told him they planned to indict him.

The unusual pace and aggression with which Mueller is moving against Manafort is a classic sign that the special counsel wants to coerce Manafort's cooperation in the Russia probe, wrote former federal prosecutor Renato Mariotti.

"The tactic that Mueller is using - telling Manafort that he will be charged - is generally used when prosecutors are trying to get a defendant to 'flip,'" said Mariotti. The best way to do that, he added, is to assemble enough evidence to make it likely that the individual, if charged, would be convicted and sentenced to jail time.

It's hard to tell what Mueller has on Manafort, "but they absolutely have something because they got a search warrant," said Joseph Pelcher, a former FBI counterintelligence agent who was stationed in Russia and specialized in organized crime. "You need probable cause to get a search warrant, so there is something there, without question."

"If I were investigating the case, the first thing I would do is sit Manafort down and get him to cooperate, because he's not the big fish here," Pelcher added.

It's unclear whether Manafort has any incriminating information on Trump, who is a focus in at least one thread of Mueller's investigation.

The special counsel is reportedly building an obstruction-of-justice case against the president in the wake of his decision to fire former FBI director James Comey in May. At the time, Comey was spearheading the FBI's investigation into whether the Trump campaign colluded with Russia.

Though the White House initially said Comey was fired because of his handling of the Clinton email investigation, Trump later told NBC's Lester Holt that "this Russia thing" had been a factor in his decision.

Mueller is also scrutinizing Trump's role in crafting a misleading statement his son, Donald Trump Jr., issued in response to revelations that he met with a Russian lawyer offering dirt on Hillary Clinton at Trump Tower last June. Manafort was present at the meeting, as was Trump's son-in-law and senior adviser Jared Kushner.

If Mueller believes Trump did something wrong, his laser focus on Manafort is likely an attempt to "bargain for whatever information he might have," said Pelcher. Even if the former campaign chairman doesn't have incriminating evidence against Trump, Manafort could still possess a trove of other valuable information.

If Manafort was in touch with the Russians during the campaign, Mueller can press him for information about how they operated, who they were in contact with, what channels they used, and more. "There's a lot Mueller could unpack there, even from an intelligence angle," Pelcher said.