WASHINGTON: Investigators Tuesday scoured the wreckage of two commuter trains hunting for clues to the worst metro accident in Washington's history, which killed nine people and injured 76 others. Metro officials, who have operated the US capital's busy network for 33 years, said they still had no clue why one train ploughed into the back of another on an above ground section of the line just at the start of Monday's rush hour. Two more passengers died Tuesday from their injuries, pushing the death toll to nine. The dead include driver, Jeanice McMillan, 42, whose train slammed into the other one with such force that it thrust two carriages from the front one up onto her train.Officials from the National Transportation Safety Board were proceeding with a painstaking probe of Monday's accident which turned a routine commute into a nightmarish disaster scene. "Our first thoughts and first efforts here in DC are with the families and friends of the victims," Washington Mayor Adrian Fenty said Tuesday in the aftermath of the collision he described as "harrowing." Metro General Manager John Catoe said in a statement: "We are committed to investigating this accident until we determine why this happened and what must be done to ensure it never happens again." The statement added that it was still not known what caused the crash, and the investigation could take "several weeks or months."