Special counsel Robert Mueller says former Trump campaign chair Paul Manafort broke his plea deal by lying to investigators.

In a court filing Monday, Mueller alleged that Manafort "committed federal crimes" by lying about "a variety of subject matters" — which could result in Manafort receiving a longer prison sentence.

Homeland security expert Juliette Kayyem joined Boston Public Radio to examine Manafort's motivations.

"Manafort has a lot of friends and therefore a lot of enemies," Kayyem said. "He is a man without nation who has basically defended and represented the worst of the worst. That includes people who may not be too happy with any of the financial disclosures."

Kayyem said there are two theories about why the former Trump campaign chairman may be lying to investigators.

"One theory is the domestic one, which is: He's lying because he's hoping for a pardon from Trump and does not want to throw Trump or the family under the bus, so he continues to lie despite this offer that Mueller gave him," Kayyem said.

But there is another theory, too: that Manafort may be protecting his allies outside the United States. On Thursday, the Wall Street Journal reported that Manafort's alleged misstatements to Mueller "include comments about his personal business dealings and about his contacts with a former associate in Ukraine."

"The other [theory] is they might have information about foreign contacts, of which those people are not very nice people and he might have lied to protect himself or to protect them," Kayyem said.