WASHINGTON: The probe into Robin L. Raphel began after US investigators intercepted a conversation in which a Pakistani official suggested that his government was receiving American secrets from a prominent former State Department diplomat, the New York Times quoted officials as saying.

The conversation set off an espionage investigation that has caused a flurry in US diplomatic circles.

In the wake of the conversation, months of secret surveillance was carried out on former diplomat Robin L. Raphel along with an Federal Bureau of Investigation (FBI) raid last month at her home, where agents discovered secret information, officials said.

Ms Raphel, 67, who served as an envoy to Tunisia and as an assistant secretary of state for South Asian affairs during the Clinton administration, became one of the highest-ranking female diplomats and a fixture in foreign policy circles. The investigation proved to be an unexpected turn in a career that has spanned four decades.

Explore: Analysis: Diplomat in trouble

Raphel is considered one of the leading American experts on Pakistan. She was stripped of her security clearances last month and no longer has access to the State Department building.

At a time when Pakistan was one of the closest allies of the US and a safeguard against the Soviet Union, many diplomats, including Raphel, rose through the ranks of the State Department.

After Raphel retired from the government in 2005, she lobbied on behalf of the Pakistani government before accepting a contract to work as a State Department adviser.

Take a look: Robin Raphel probe internal matter of US, says FO