Long before the special counsel began investigating Russian meddling in the U.S. election -- and whether anyone in the Trump campaign was involved -- Paul Manafort was the target of an FBI investigation. It included electronic surveillance of the one-time Trump campaign chairman.

CBS News has learned that the surveillance on Manafort occurred during the 2016 presidential campaign.

According to a former U.S. official, the intercepts picked up conversations between Manafort and Russian individuals about the campaign. The intercepts potentially include conversations between Manafort and President Trump.

The recordings are now part of special counsel Robert Mueller's investigation, which appears to be intensely focused on Manafort.

Former Trump campaign chairman Paul Manafort. CBS News

Just last week, Manafort's spokesman Jason Maloni appeared before a grand jury. In July, agents raided Manafort's condominium in Virginia by reportedly picking the locks to get in while he was asleep.

Peter Zeidenberg, a former federal prosecutor, told CBS News he would characterize what is happening with Manafort as "very aggressive." He says investigators may be pressuring Manafort to give up information in exchange for leniency.

Zeidenberg said prosecutors do "exactly what they're doing now" to Manafort to get someone to cooperate.

"Search warrants, interviewing everybody around him and then advising him that he's about to be or will be indicted," Zeidenberg said.

In a statement, a spokesman for Manafort called on the Department of Justice to "release any intercepts involving [Manafort] and any non-Americans," claiming that there is "nothing there."

Manafort has been under legal pressure before. In 1989, he was accused of being part of a scheme to steer millions of dollars to well-connected Republican Party operatives through a New Jersey housing project.

During a congressional hearing, before being cleared of any wrongdoing, the well-connected lobbyist was pressed to describe what his job entailed.

"You could characterize this as influence peddling," Manafort said.

Also on Tuesday, Senate Intelligence Committee leaders abruptly canceled a closed meeting between Mr. Trump's attorney Michael Cohen and committee investigators, scolding Cohen for releasing a public statement in advance. The committee later invited Cohen to appear at a public hearing next month.