The former US envoy to Ukraine was put under surveillance, texts unveiled as evidence for President Donald Trump's impeachment trial suggest.
The messages indicate an associate of Mr Trump's lawyer Rudy Giuliani received updates on ambassador Maria Yovanovitch's location and phone use.
The US House of Representatives has released a trove of documents to bolster their case against Mr Trump.
The president is accused of abuse of power and obstruction of Congress.
Mr Trump was impeached by the House last month. He denies trying to pressure Ukraine to open an investigation into his would-be Democratic White House challenger Joe Biden.
House Democrats are voting on Wednesday to send the articles of impeachment to the Senate, which will hold the president's trial.
Democrats said they would also send over the new evidence unveiled on Tuesday.
House Intelligence Committee chairman Adam Schiff, who will lead the prosecution of Mr Trump, said the documents "demonstrate that there is more evidence relevant to the president's scheme, but they have been concealed".
What is the new evidence?
The letters, phone records, notes and flash drives were obtained from the Ukrainian-American businessman Lev Parnas, a close associate of Mr Giuliani, who is the US president's personal lawyer.
They were made available to investigators earlier this week and then sent to the House Judiciary Committee.
The documents show that Ukraine-born Mr Parnas was in regular contact with Mr Giuliani as well as Ukrainian officials, and suggest that Mr Parnas was directly involved in trying to have Ukraine's President Volodymyr Zelensky look into Mr Biden.
One handwritten note from Mr Parnas, who was indicted last year on conspiracy charges, states: "Get Zalensky [sic] to Annouce [sic] that the Biden case will be investigated."
Also among the new materials is a screenshot of a previously undisclosed letter from Mr Giuliani to Mr Zelensky, in which he asks to arrange a meeting.
The letter from Mr Giuliani describes himself as "personal counsel to President Trump" and states that Mr Trump had "knowledge and consent" of Mr Giuliani's actions.
The meeting never took place as Mr Giuliani eventually cancelled his May trip to Ukraine.
Was the envoy under surveillance?
Some of the materials also show Mr Parnas and Mr Giuliani discussing the removal of Ms Yovanovitch, the then US ambassador to Ukraine.
Several of the text messages appear to suggest the former US envoy was placed under surveillance.
Mr Parnas was apparently given updates on the ambassador's location and mobile phone use by a man named Robert F Hyde.
Mr Hyde is a Republican congressional candidate in Connecticut and a Trump campaign donor.
"She's talked to three people. Her phone is off. Computer is off," one message reads. "They will let me know when she's on the move," another says.
Another message from Mr Hyde to Mr Parnas said: "They are willing to help if we/you would like a price. Guess you can do anything in the Ukraine with money... what I was told."
Ms Yovanovitch is calling for an investigation into the messages.
"The notion that American citizens and others were monitoring [her] movements... is disturbing," her lawyer said.
Who is Marie Yovanovitch?
The veteran envoy was recalled from Ukraine in May 2019 for reasons that remain unclear.
She testified to the Trump impeachment inquiry last October that she was fired over "false claims" by people with "questionable motives".
She also said US state department officials had told her she needed to leave Ukraine immediately out of concerns for her "security".
President Trump complained following her testimony that she had refused to hang his photo in the US embassy in Ukraine.
Want to find out more?
- A SIMPLE GUIDE: If you want a basic take, this one's for you
- GO DEEPER: Here's a 100, 300 and 800-word summary of the story
- A STATE DIVIDED: What New Hampshire makes of it
- YOUR QUESTIONS: Will Trump really testify?
- HISTORY: Can an impeached president remain popular?
- CASE FOR & AGAINST: What legal scholars say about Trump conduct
- WHAT'S IMPEACHMENT? A political process to remove a president