Investigators began searching through the wreckage of Beirut's port Wednesday for clues to the cause of the massive explosion that ripped across the Lebanese capital, and the government ordered port officials put under house arrest amid speculation that negligence was to blame.

The investigation is focusing on how 2,750 tons of ammonium nitrate, a highly explosive chemical used in fertilizers, came to be stored at the facility for six years, and why nothing was done about it.