Updated

The Washington Navy Yard began returning to nearly normal operations three days after it was the scene of a mass shooting in which a gunman killed 12 people.

The Navy installation re-opened 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.

Law enforcement officials are still trying to determine a motive for the shooting. Officials have said the 34-year-old gunman was grappling with paranoia, hearing voices and convinced he was being followed. A month before the shootings, he complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel room and sending microwave vibrations into his body to deprive him of sleep.

On Wednesday, the Department of Veterans Affairs told lawmakers in Congress that Alexis visited two VA hospitals in late August complaining of insomnia, but that he denied struggling with anxiety or depression or had thoughts of harming himself or others. On Aug. 23 he visited an emergency room at the VA Medical Center in Providence, R.I. He made a similar visit five days later to the VA hospital in Washington.

Also on Wednesday, families began claiming the bodies of their loved ones from the medical examiner's office in Washington.