President Barack Obama's spy chief James Clapper raised the possibility that impeachment was a factor behind U.S. Attorney John Durham opening a criminal inquiry into the origins of the Trump-Russia investigation.

Clapper, a former director of national intelligence who is a focus of the DOJ investigation, told CNN anchor Anderson Cooper he learned of the criminal investigation 20 minutes prior by reading a news article, and it made him "very curious."

He suggested there was a political motivation behind it by noting how President Trump is facing intensifying impeachment proceedings. "I found the timing interesting, given the increasing heat around the impeachment inquiry. And so the timing is interesting. I’ll just let it go at that," Clapper said Thursday evening.

Clapper and former CIA Director John Brennan, who is also a subject of interest in the Durham investigation, led the Obama administration's intelligence agencies at a time when some Republicans allege the CIA illicitly ensnared members of the Trump campaign using informants. They anticipate the DOJ inquiry will be very critical of Clapper and Brennan.

[Read more: John Ratcliffe: FISA report will show why DOJ opened criminal inquiry into Russia investigation]

Yet aspects of Durham's secretive investigation, which Democrats worry is a "vehicle for President Trump's political revenge," appeared to mystify Clapper.

"The other thing I wonder about is whether we’re talking about the overall investigation of the Russian — reporting on the Russian interference — or are we talking about the counterintelligence investigation that was launched in July [2016] by the FBI about potential engagement into collusion, whatever you want to call it, between the Russians and the Trump campaign," he said.

Asked whether he has any idea what Durham's team may think rises to the level of a criminal offense, Clapper said he had no clue. "That’s obviously an item of great interest to me. What is it that any of us did that would rise to the level of a criminal infraction? I just don’t know," he said.