Special Counsel Robert Mueller issued grand jury subpoenas seeking testimony from public relations executives who worked on a lobbying campaign organized by Paul Manafort, including the Podesta Group.

The Podesta Group, co-founded by former chairman of Hillary Clinton’s campaign John Podesta, was one of six lobbying firms who worked on Manafort’s campaign between 2012 to 2014 to get the Ukraine into the European Union, NBC News reported.

“One executive whose firm received a subpoena” told NBC News that Mueller’s team is closely examining the lobbying campaign that the Podesta Group was part of.

“We think they are trying to figure out, was this a legitimate project?” the unnamed executive said. “From our perspective it was — we did a lot of work. We took it seriously.”

The executive said six firms participated in Manafort’s campaign, which was paid for by a Brussels-based non-profit called the European Center for a Modern Ukraine. The stated goal was to build support for Ukraine’s entry into the European Union, NBC News reported.

Two of the firms, Podesta Gorup and Mercury LLC, worked with Manfort partner Rick Gates, according to lobbying disclosure records, NBC News reported. Three other firms worked in Europe.

At the time of Manafort’s campaign, Ukraine was run by a pro-Russian political party that paid Manafort $17 million for consulting in 2013 and 2014, according to a lobbying disclosure filing.

Manafort’s spokesman, Jason Maloni, declined to comment to NBC News.

At the time, neither Manafort nor the firms had registered as foreign agents. The unnamed executive told NBC News it was because the non-profit said that it was not funded by a government or a political party.

In recent months, the Justice Department told the Mercury and Podesta firms that they should have registered as foreign agents, and they did so, according to NBC News.