House Democrats have released a trove of documents they obtained from a close associate of President Donald Trump's personal lawyer Rudy Giuliani, including a handwritten note that mentions asking Ukraine's president to investigate "the Biden case."

The documents were obtained as part of the impeachment investigation, and show Lev Parnas communicating with Giuliani and Republican attorney Robert Hyde about the removal of the U.S. ambassador to Ukraine, Marie Yovanovitch.

Parnas received messages from a man who appeared to be describing detailed surveillance of Yovanovitch while she was in Ukraine.

In the text messages, Hyde refers to Yovanovitch as a "b****" and shares Yovanovitch's location and level of security around her.

"It's confirmed we have a person inside," Hyde said in a text.

Following the release of the text messages on Tuesday, Yovanovitch called for an investigation, saying the fact that she was being surveilled was "disturbing."

"Needless to say, the notion that American citizens and others were monitoring Ambassador Yovanovitch's movements for unknown purposes is disturbing," said Lawrence S. Robbins, Yovanovitch's attorney, in a statement, according to CNN. "We trust that the appropriate authorities will conduct an investigation to determine what happened."

The documents released Tuesday add new context to Democrats' charges that Trump pressured Ukraine to investigate Joe Biden and other Democrats as Trump withheld military aid.