US investigators obtained a warrant to wiretap 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.

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.

Former FBI agent Asha Rangappa, who served in the bureau’s counterintelligence division, explained earlier this year the FBI can request and be granted a warrant to monitor a US person from the Foreign Intelligence Surveillance Court “if they can show probable cause that the target is an ‘agent of a foreign power’ who is ‘knowingly engag[ing]…in clandestine intelligence activities.'”

“In other words,” Rangappa added, “the government has to show that the target might be spying for a foreign government or organization.”

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Manafort was previously surveilled under a separate FISA authorization that began in 2014, due to scrutiny over his lobbying work on behalf of the pro-Russia Party of Regions in Ukraine and his business dealings with Russian entities. That surveillance ended due to a lack of evidence, according to CNN, but was later restarted under the new warrant that extended into 2017.

Information obtained from the newly discovered FISA warrant was shared with Mueller’s team, CNN said.

The government eavesdropping apparently 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 July, in search of tax documents and foreign banking records. Mueller threatened to indict Manafort following the raid, according to The Times.

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.

Manafort’s international work has long raised eyebrows among Democrats in Washington. In 2004, he became a top adviser to Ukrainian President Viktor Yanukovych, a pro-Russian strongman whom Manafort is widely credited with helping win the presidency in 2010.

Yanukovych was ousted in 2014 after widespread demonstrations against his decision to back out of a deal with the EU that would have distanced Ukraine from Russia and fostered closer ties with the West. On February 20, 2014, Ukrainian riot police opened fire on thousands of demonstrators who had gathered in central Kiev. Fifty-three protesters were killed that day, and dozens more over the next few days.

Ukrainian prosecutors have said Yanukovych ordered the security forces’ attack on protesters, and at least one human-rights lawyer representing the victims is investigating what role, if any, Manafort played in encouraging Yanukovych’s crackdown. Yanukovych fled to Russia amid the protests and is now living under the protection of the Kremlin.

Manafort served as Trump’s campaign chairman until August 2016, when he resigned over reports that the Party of Regions had earmarked him $12.7 million in undisclosed cash payments for his work between 2007 to 2012. Manafort has denied ever collecting those payments.