The US military has called for "vigilance" after an online threat was allegedly made by Islamic State (IS) to about 100 of its personnel.
A list of names and addresses was posted on a website linked to the group alongside a call for them to be killed.
The Pentagon said the threat was unverified and would be investigated.
The IS-linked group said it obtained the information by hacking servers and databases, but US officials said most of the data was in the public domain.
A US security source told the BBC that those on the list were being contacted.
The group, which called itself the Islamic State Hacking Division, said the personnel named had participated in US missions against IS.
It urged its supporters in the US to "take the final step" and "deal with" those named.
A US defence official told US media: "I can't confirm the validity of the information, but we are looking into it."
US Marine Corps spokesman Lt Col John Caldwell said: "Vigilance and force protection considerations remain a priority for commanders and their personnel."
He recommended that marines "check their online/social footprint, ensuring privacy settings are adjusted to limit the amount of available personal information".
The US and its allies have been conducting air strikes against Islamic State militants in Syria and Iraq since last September.