A truck packed with rockets blew up Wednesday in a Shiite area of Baghdad, killing 18 people in the deadliest single blast in the city in more than three months. Three U.S. soldiers were killed by gunfire north of the capital.

A U.S. military spokesman said the blast appeared to have been an accident that occurred as Shiite militiamen were transporting the weapons through a densely populated neighborhood of northern Baghdad – possibly to fire at a nearby American base.

Iraqi police said a suicide truck bomber had targeted the house of an Iraqi police general who was not at home but whose nephew was among those killed. U.S. officials said 75 people were wounded, and police said they included the general’s elderly parents.