5:00 - A high-ranking police official tells Dam that the explosion was near TV Hill in the center of Kabul behind a house, and that there were no casualties. 

4:50 - NATO forces tell the Journal's Nissenbaum that the explosion "was criminal: one 'tribe' planting roadside bomb to target another." The independent journalist Bette Dam is calling it a night: "Ok, no news.. The explosion was impressive but aftermath frustrating.. We dont know location. Will go to sleep, early flight to the south." As of 17 minutes ago, however, van Houdt hadn't given up: "nothing @ police HQ or this side of TV Hill... Suggestions? ?"

4:41 - CNN is reporting that the explosion occurred a couple of miles southwest of the U.S. Embassy.

4:33 - A number of Kabul-based journalists are now canvassing Kabul, which has since fallen silent, in the dead of night to determine where the blast occurred, and communicating with each other via Twitter. A policeman tells photojournalist Joel van Houdt, who is driving around the city, that nothing happened at the U.S. Embassy. "Not sure if I should be amused or concerned that no one can find the site of an explosion we all heard," the Twitter user @heyosita writes. Independent journalist Erin Cunningham wonders whether, in the wake of the Osama bin Laden news and the Taliban announcing a "spring offensive," everyone is just "super jumpy."

Original Post (3:59 p.m.)

Twitter is beginning to buzz with reports of a major explosion in Kabul, Afghanistan. Jonathan Boone, a correspondent for The Guardian, tweeted around 3:30 p.m. East Coast time, "Midnight in Kabul and a huge explosion." He added, "Mystery phone caller just said bang was at south side of US Embassy..." The Wall Street Journal's Dion Nissenbaum added more recently that NATO forces are saying the explosion was west of the embassy: "ISAF says explosion hit somewhere west of ISAF HQ/US embassy."