The FBI opened a counterintelligence investigation of a veteran U.S. diplomat after intercepting the communication of a Pakistani official that indicated the woman might have been involved in providing secrets to the Pakistani government, U.S. officials said.

The intercept triggered months-long surveillance of Robin L. Raphel, a former ambassador and assistant secretary of state, and eventually led the bureau to obtain a warrant to search her home in Washington; her office at the State Department was also searched. It is unclear what form of communication was intercepted by the United States.

One U.S. official, speaking on condition of anonymity to discuss an ongoing case, said the bureau found classified information at Raphel’s home.

The New York Times first reported on the intercept and the lengthy surveillance of Raphel, 67, who has not spoke publicly since news of the investigation broke this month.

“Ambassador Raphel is a highly respected career diplomat who has dedicated her life to serving the United States and its interests,” said Amy Jeffress, Raphel’s attorney and the former chief of the National Security Section in the U.S. Attorney’s Office for the District of Columbia. “She would never intentionally do anything to compromise those interests. She, and we as her counsel, are cooperating with the investigation, and we are confident that she will be cleared of any suspicion.”

Robin Raphel testifes during a Senate Foreign Relations Committee Hearing on Capitol Hill, April 7, 2004 in Washington. (Mark Wilson/Getty Images)

A spokesman for the Justice Department would neither confirm nor deny the existence of an investigation.

Raphel’s security clearances were pulled and her contract with the State Department was allowed to expire because of the investigation.

The search of Raphel’s home was one of several open steps by the FBI that indicated the investigation was at an advanced stage and agents were comfortable with some details about the case becoming public.

Many FBI counterintelligence investigations never result in criminal charges. Espionage cases involving Americans are rare and often difficult to prosecute because they involve classified material the government does not want made public.

Raphel is a well-known figure in diplomatic circles She began her government career as a CIA analyst, according to a State Department biography. She served 30 years in the Foreign Service and retired from the State Department in 2005.

Her former husband, Arnold L. Raphel was the U.S. ambassador to Pakistan when he was killed in a 1988 plane crash with the president of Pakistan, Gen. Mohammad Zia ul-Haq. Questions about whether the crash was in fact an assassination continue to swirl around the incident.

Raphel returned to the State Department in 2009 to work as an adviser to Richard Holbrooke, who had been named by then-Secretary of State Hillary Rodham Clinton to the new post of special representative for Afghanistan and Pakistan.

Raphel also worked as a lobbyist for Cassidy & Associates, a Washington-based government relations firm, before returning to the government. As part of that job, she represented Pakistan.

Carol D. Leonnig contributed to this report.