The FBI has interviewed Syrian activists in Washington and expressed concerns about their safety, according to local opponents of the rule of Syrian President Bashar al-Assad.

After initial meetings that began several weeks ago, FBI agents have maintained contact with key activists, one of whom says she received a telephone death threat in May.

Hala Abdul Aziz, a Syrian American who lives in Alexandria, said a caller warned her that her daughter in Syria would “vanish” if Abdul Aziz continued to press a U.S. civil suit that accuses Syrian officials of human rights abuses.

Activists in Washington, like their compatriots in Europe and the Middle East, have been instrumental in helping to disseminate information about ongoing protests against Assad’s rule. They have also mounted protests of their own and lobbied Western governments and international organizations on behalf of dissidents in Syria.

The State Department, citing reports that Syrian Embassy officials had carried out surveillance of protesters in the United States, summoned Ambassador Imad Moustapha on July 6. In a statement two days later, the department also said it was “investigating reports that the Syrian government has sought retribution against Syrian family members for the actions of their relatives in the United States.”

Moustapha met with a U.S. official in early July but was “presented with no evidence” of such activities, said an official at the embassy who spoke on condition of anonymity. “No one at the embassy in their official capacity would call [activists] and threaten them,” said the official, who was not authorized to speak publicly.

Radwan Ziadeh, a Syrian visiting scholar at George Washington University and a critic of Assad’s government, received a call from the FBI about three weeks ago.

“They had some concerns about the families of the activists in America,” said Ziadeh, who said family members in Syria has been approached by the security forces and asked to disown him and his work.

Ziadeh said that the agent discussed the situation in Syria and has remained in contact with him.

An FBI spokesman declined to comment on the inquiries, saying the bureau doesn’t confirm the existence of an investigation or comment on its activities.

Abdul Aziz, who said her father was fatally shot by government security forces in the southern Syrian city of Deraa in April, said she contacted the FBI through her lawyer after receiving a call from a man who threatened her daughter and other family members.

“I assumed that the threat came from the embassy,” Abdul Aziz said. She and other plaintiffs filed a lawsuit in May seeking damages from the Syrian government, drawing television news coverage.

She met twice with an FBI agent, who said that he would look into who made the call, and who has been in touch with her by telephone several times since.

The FBI has called and met with other prominent Syrian and Syrian American activists in recent weeks, asking questions about the situation in Syria and their roles in the opposition movement and voicing concern for their safety.

Sirwan Kajjo, a Syrian Kurd who has protested many Saturdays outside the White House and the Syrian Embassy since unrest gripped his homeland in March, said that he had met with an FBI agent this month.

Accompanied by other activists, he discussed his role in the Syrian uprising and was offered assurances of protection.

“I would say the main purpose of the meeting was to show support,” Kajjo said.

Staff writer Joby Warrick contributed to this report.