Washington(CNN) Special Counsel Robert Mueller has issued subpoenas to a former lawyer for Paul Manafort and to Manafort's current spokesman, an aggressive tactic that suggests an effort to add pressure on the former Trump campaign chairman.

The subpoenas seeking documents and testimony were sent to Melissa Laurenza, an attorney with the Akin Gump law firm who until recently represented Manafort, and to Jason Maloni, who is Manafort's spokesman, according to people familiar with the matter.

Manafort is under investigation for possible tax and financial crimes, according to US officials briefed on the investigation. The allegations under investigation largely center on Manafort's work for the former ruling party in Ukraine, which was ousted amid street protests over its pro-Russian policies.

It's unclear what specific information the Mueller investigators believe Laurenza and Maloni may have. But issuing subpoenas to a lawyer of someone under investigation is unusual, in part because it raises potential attorney-client privilege issues that prosecutors tend to try to avoid. Maloni, as a public relations representative, doesn't have the same attorney-client privilege protections.

Subpoenas have not been sent to WilmerHale, Manafort's other most recent lawyers. WilmerHale is the law firm where Mueller worked before being appointed special counsel.

Manafort has not been accused of any wrongdoing.

Maloni declined to comment. A spokesman for Mueller declined to comment. Laurenza referred questions to a spokesman who didn't immediately comment.

The subpoenas are among dozens that the Mueller investigative team has sent out in recent months since taking over the investigation into Russian meddling in the 2016 presidential election.

The Manafort investigation has been ongoing for years and as recently as last summer appeared to be stagnated. At the time, Justice Department officials had come to believe there wasn't enough evidence to pursue charges, as CNN has previously reported.

The probe has taken on new life since the beginning of this year, and appears to be taking an aggressive turn under the Mueller investigators.

Andrew Weissmann, who is one of the senior lawyers overseeing the investigation, is known for aggressive tactics. Last month, the Mueller team ordered a dawn raid on Manafort's home in Washington's Virginia suburbs, as Manafort slept in his bedroom. The financial documents retrieved were largely ones Manafort had already turned over to congressional investigators.

Manafort is now represented by Kevin Downing, a Washington lawyer and former Justice Department prosecutor who specializes in tax issues.