Some Republican senators are expanding their investigation into Hunter Biden and his one-time Ukrainian employer Burisma Holdings by requesting interviews and records from former Obama administration officials.

Sens. Chuck Grassley (R-IA), Lindsey Graham (R-SC), and Ron Johnson (R-WI) made the announcement on Wednesday as House Democrats prepared to impeach President Donald Trump in part because of the younger Biden’s overseas work. The senators — who chair the Senate Homeland Security, Finance, and Judiciary Committees respectively — are looking into whether Hunter Biden and the Ukrainian natural gas company Burisma benefited from exclusive access to the Obama administration.

As part of the investigation, the senators now plan to interview former top aides to former Vice President Joe Biden and ex-Secretary of State John Kerry. Included on the list are former U.S. Deputy Secretary of State Anthony Blinken, former international energy affairs envoy Amos Hochstein, ex-Under Secretary of State Catherine Novelli, former Assistant Secretary of State Victoria Nuland, and one-time Kerry chief of staff David Wade.

Documents revealed through a Freedom of Information Act request in November showed that officials like Blinken, Hochstein, and Novelli met with Burisma lobbyists in 2015 and 2016. The meetings were arranged after the lobbyists name-checked Hunter Biden when reaching out to the Obama-era officials.

Graham, in particular, has cited the timing of those meetings and the pressure campaign that followed — in which Joe Biden demanded Ukraine fire its top prosecutor, who at the time was looking into Burisma for corruption — as an impetus for further investigation.

The GOP senators’ decision to expand their investigation by interviewing the former Obama administration comes as Hunter Biden and his father struggle to explain why he was appointed to Burisma’s board of directors in the first place.

As Peter Schweizer detailed in Secret Empires: How the American Political Class Hides Corruption and Enriches Family and Friends, the younger Biden secured the position, which paid as much as $83,000 per month, despite having no background in energy or Ukraine.

Adding to concerns was that Hunter Biden joined the company at a time when it was actively courting western leaders to prevent scrutiny of its practices. The same month as the appointment, Mykola Zlochevsky, Burisma’s founder, had his assets frozen in the United Kingdom on suspicion of money laundering.

A Ukrainian official with ties to Zlochevsky admitted in October the only reason Hunter Biden secured his position with Burisma was to “protect” the company from foreign scrutiny.