From TheAtlantic.com:

Forces loyal to Libyan leader Muammar Qaddafi captured four journalists on Tuesday at around 1 p.m. local time a few miles outside of Brega, which is currently under government control. One of the reporters, Clare Morgana Gillis, has reported extensively for TheAtlantic.com from Libya during the civil war. Several Libyan rebels witnessed loyalist troops stop a car carrying the journalists at an intersection where there had been recent fighting. The troops took the four journalists into custody, released the driver, and destroyed the car with an RPG. The rebels later recounted the incident to reporters with The New York Times, who alerted Human Rights Watch in turn.

Max Fisher has more detail on the situation, including Atlantic editor James Bennet's statement: "We appeal to the Libyan authorities for her immediate and safe release, and for that of the three other journalists detained with her."