US investigators reportedly wiretapped Paul Manafort before and after the election

US investigators spied on former Trump campaign chairman Paul Manafort before and after the election, CNN reported Monday night.

The revelation marks a potentially significant development in the ongoing probe into potential collusion between the Trump campaign and Russian operatives in the 2016 presidential election.

Some of the information gleaned from the surveillance prompted concerns that Manafort had encouraged Russians to "help with the campaign," CNN reported, citing three unnamed sources.

Investigators last year obtained a Foreign Intelligence Surveillance Act (FISA) warrant to conduct surveillance on Manafort, which continued through early 2017.

Manafort was previously surveilled under a separate FISA authorization that began in 2014, due to scrutiny over his extensive ties to pro-Russia groups and work he has done on their behalf in previous years. Citing one of its sources, CNN reported that the surveillance ended due to a lack of evidence, but it was later restarted under the new warrant that extended into 2017.

CNN said the government spying included a period earlier this year when Manafort was "known to talk" to President Donald Trump. The president sparked weeks of speculation in March when he accused former President Barack Obama of having his "wires tapped" in Trump Tower. Trump's own Justice Department, however, has said there is no evidence to support Trump's claim.

Manafort was forced to register as a foreign agent in April.

The focus on Manafort has only increased since special counsel Robert Mueller took over the investigation in May, shortly after Trump fired FBI Director James Comey. Since then, the FBI has also conducted a raid on one of Manafort's homes in August, in search of tax documents and foreign banking records, according to The New York Times. Information obtained from the newly discovered FISA warrant was shared with Mueller's team, CNN said.

New York Attorney General Eric Schneiderman was recently recruited to help investigate Manafort for possible financial crimes and money laundering. The IRS's criminal-investigations unit has been brought onto the investigation to examine similar issues.

Natasha Bertrand contributed reporting.