WASHINGTON (AP) — The Washington Navy Yard is reopening to workers three days after it a mass shooting in which a gunman killed 12 people.

Gates at the Navy installation reopened at 6 a.m. Thursday.

But traffic was blocked from reaching the main gate because a tractor-trailer tried to make a U-turn, and its load shifted. The truck was blocking the road in front of the Navy Yard early Thursday.

Navy spokeswoman Lt. Cmdr. Sarah Flaherty said Thursday will be a regular work day, except for Building 197, where the shootings occurred, and the base gym. She says the gym is being used as a staging area for the FBI to investigate Monday's rampage in which former Navy reservist Aaron Alexis gunned down 12 people before being killed by police.