Esam Al-Fetori / REUTERS
The Americans were in Libya "augmenting security at the U.S. Embassy in Tripoli," the official told Starr.
The circumstances were not immediately clear, but Fox News confirmed four Americans were in custody. "We are seeking to further ascertain the facts and ensure their release," State Department spokesman Jen Psaki told Fox News. "We are in touch with Libyan officials on this issue."
The episode appears to have taken place in a town just southwest of the historic Roman ruins at Sabratha and about an hour's drive from Tripoli, the capital. The area is not known for anti-Western extremists or other obvious threats. In part because it is a tourist area, the district around Sabratha skews relatively liberal and friendly to Westerners.
The embassy in Tripoli issued a security message in November, advising U.S. citizens to avoid large gatherings and "maintain a high level of vigilance."
The security posture in Libya was ramped up substantially in the wake of an attack on the U.S. mission in Benghazi, where four Americans were killed on Sep. 11, 2012.
This is a developing story, and will be updated as we learn more details.