WASHINGTON – The contract worker who opened fire at the Washington Navy Yard this week appeared to have no particular target as he moved through a building and shot and killed 12 people, FBI Director James Comey said on Thursday.

Comey, whose agency is leading the investigation into the shooting, said that in surveillance video the man identified as 34-year-old Aaron Alexis "appears to be moving without particular direction or purpose."

Thousands of workers streamed back into the Washington Navy Yard on Thursday, three days after Alexis, a former reservist working at the site as a contractor, opened fire with a shotgun as he wandered several floors and hallways.

The sprawling, walled complex, which covers about 16 blocks of the U.S. capital, had been closed to all but essential personnel and those involved in the investigation into why Alexis, who died in a gun battle with police, mounted his attack.

Three people were wounded and one, a woman shot in the head and hand, was discharged on Tuesday, MedStar Washington Hospital Center said in a statement. The other two, a Washington police officer with leg wounds and a woman shot in the shoulder, are in good condition, it said.

Comey told reporters in a briefing at FBI headquarters that he would not comment on a possible motive for the shooting but said investigators continued to examine evidence, including Alexis' mental condition.

It was not clear how Alexis hid the shotgun that was his primary weapon, Comey said. Video shows Alexis entering a bathroom on the building's fourth floor with a bag and then emerging with the shotgun, but investigators do not know if the shotgun was disassembled, he said.

"The shotgun was cut down at both ends," Comey said. "The stock was sawed off, and the barrel was sawed off a little bit."

Alexis also fired a semi-automatic pistol that he took from a security guard he shot, Comey said. Alexis died after exchanging gunfire with law enforcement officers, about 30 minutes after he began shooting.

Alexis, a U.S. Navy Reserves veteran, entered the base on Monday with a security clearance that allowed him onto military facilities to work as an information technology contractor.

He had been working on a computer server project in the same building where he launched the attack, Comey said.

Heard voices

The Department of Veterans Affairs said on Wednesday that Alexis was treated for insomnia in August at hospitals run by the VA, but that he said he did not have violent thoughts and did not seek care from a VA mental health specialist.

His credentials were still valid, although Rhode Island police had warned the Navy in August that Alexis had reported "hearing voices" and said he believed people were following him and "sending vibrations into his body," according to a Newport police report.

A senior Navy official said on Thursday the service was reviewing why that report had not been shared more widely.

"We don't have any indications that the reports from the Newport police went any higher than the local base security officers there at Newport," Rear Admiral John Kirby, Navy chief of information, told CNN.

"We're taking a hard look at that as well to see why that report didn't go any higher and what should have been done better to make authorities at a higher level than their base security aware of it," Kirby said.

The attack was the second mass shooting on a military base inside the United States in four years. In November 2009, a gunman killed 13 people and wounded 32 at the Fort Hood Army base in Texas.

That shooter, Major Nidal Hasan, said the attack was intended as retaliation for U.S. wars in the Muslim world. Last month a military jury sentenced him to death.

Both incidents shed light on a security condition within military installations, where military personnel other than security forces are typically prohibited from carrying firearms.

After Monday's attack, Defense Secretary Chuck Hagel ordered a worldwide review of security at U.S. military installations.

The same company that scrutinized former spy agency contractor Edward Snowden for a U.S. government security clearance said on Thursday it also checked the background of Alexis, allowing him to obtain a "secret" clearance.

USIS, working as a contractor for the Office of Personnel Management (OPM), conducted a background review of Alexis in 2007, but declined to provide more information on the grounds that it was contractually barred from retaining case data from such checks.

Earlier this year, USIS became the focus of congressional scrutiny when it was disclosed that the company handled the background investigation of Snowden, accused of disclosing top secret materials taken from the National Security Agency facility where he worked.

USIS does about 65 percent of all background check investigations conducted by government contractors and more than half of those conducted by the OPM, according to Senator Claire McCaskill, who heads the Senate Subcommittee on Financial and Contracting Oversight.

"From Edward Snowden to Aaron Alexis, what's emerging is a pattern of failure on the part of this company, and a failure of this entire system, that risks nothing less than our national security and the lives of Americans," McCaskill said in a statement on Thursday.

USIS has been undergoing an investigation by OPM's inspector general since before the Snowden and Alexis incidents. OPM's inspector general's office had no comment.

Washington Navy Yards, on the Anacostia River, was established in 1799 as a shipbuilding site and a base for defending the city. The complex houses the Naval Sea Systems Command, as well as a human resources operation for Navy civilian workers and a museum and art gallery.