A suspected Russian spy worked at the US embassy in Moscow for a decade before being quietly dismissed last year, reports said Thursday.
The woman, a Russian national, was hired by the Secret Service and came under suspicion following a routine security sweep carried out by the State Department, according to sources quoted by the Guardian, which broke the story, and CNN.
The probe found she was having regular unauthorized meetings with the main Russian intelligence agency, the FSB.
"We figure that all of them are talking to the FSB, but she was giving them way more information than she should have," an official told CNN.
The woman had access to the Secret Service's intranet and email systems, the reports said, giving her a window into potentially sensitive data including the schedules of the US president and vice president.
But "she did not have access to highly classified information," the source told CNN.
The Guardian meanwhile reported the Secret Service attempted to contain the embarrassment by letting her go when Russia ordered the removal of 750 personnel from the American embassy during a diplomatic spat that followed allegations of Moscow's interference in the 2016 US presidential election.
"The Secret Service is trying to hide the breach by firing [her]," a source told the British newspaper.
"The damage was already done but the senior management of the Secret Service did not conduct any internal investigation to assess the damage and to see if [she] recruited any other employees to provide her with more information."
Relations between the US and Russia have been particularly fraught since the election of Donald Trump, despite the president's personal warmth with his counterpart Vladimir Putin.
The Cold War era rivals are also deeply divided on issues ranging from the conflict in Syria to the annexation of Crimea from Ukraine and the Iran nuclear deal.