House investigators want Vice President Mike Pence to give them documents that could shed light on whether he helped President Donald Trump pressure Ukraine to investigate Joe Biden.

In a letter to Pence on Friday, the Democratic chairmen of three House committees cite reports that a Pence aide may have listened to the July phone call in which Trump pushed Ukraine’s president to investigate unfounded charges that Biden was involved in corrupt activities there.

They say they also want to learn more about Pence’s Sept. 1 meeting with President Volodymyr Zelenskiy.

The letter says there are “questions about any role you may have played in conveying or reinforcing the President’s stark message to the Ukrainian President.”

Former Vice President Joe Biden is seeking the Democratic presidential nomination.