Navy officials said that because of his disciplinary problems as a sailor, they had considered giving Alexis something less than an honorable discharge. But since he had never been convicted of a crime and had glowing fitness reports, they eventually granted him an honorable separation.

In evaluations from 2007 to 2011, first reported by Fox News, Alexis was described as "an eager trainee" with "unlimited potential" and a "get-it-done attitude." A 2008 report called him a "talented technician" on aircraft electrical systems who should be promoted.

An arrest in Georgia in 2008 for disorderly conduct stemming from an incident at a bar, however, prompted a negative review.

"He has had a severe lapse in judgment on a number of occasions and has been counseled several times for inappropriate conduct," the review said in spring 2009, noting that he had been reduced in rank and had his pay docked.

FULL COVERAGE: Navy Yard shooting

When Alexis appealed the punishment, it was overturned. Subsequent reviews were once again positive.

The system of background checks for gun purchases also failed to pick up Alexis' problems.

J. Michael Slocum, an attorney for the Sharpshooters Small Arms Range and gun store in Lorton, Va., said that "Mr. Alexis' name and other applicable information, including his state of residency, was provided to the federal [Naval Criminal Investigative Service] system, and he was approved by that system."

The shotgun purchase appears to have complied with Virginia's laws, which are less stringent than those in some other states. In 2007, after 23-year-old student Seung-hui Cho killed 32 students and faculty members at Virginia Tech, then-Gov. Tim Kaine signed an executive order requiring that the names of all people involuntarily committed to mental health facilities be provided to a federal database that licensed gun dealers are supposed to check before they sell anyone a gun.

But Alexis does not appear to have been committed. In Rhode Island, when he reported hearing voices, police told him to stay away from the people he thought were bothering him, but otherwise took no action, according to a police report released Tuesday. Rhode Island, like most other states, sets a fairly stringent standard for what officials must prove in order to involuntarily commit a person.

As a result, when Alexis reported to the Navy Yard, he "had legitimate access … as a result of his work as a contractor, and he utilized a valid pass to gain entry to the building," Valerie Parlave, assistant director in charge of the FBI's Washington field office, said at a news conference Tuesday.

Security at the base already has been reviewed. A Defense Department inspector general's report released Tuesday said that a computer database system the Navy had adopted to reduce costs while controlling contractors' access had allowed convicted felons onto bases 52 times.

Several members of Congress quickly leapt on the report, saying it pointed to problems. "This apparent security lapse — permitting people with criminal records to freely access military bases and facilities — is deeply troubling," said Sen. Richard Blumenthal (D-Conn.), a member of the Armed Services Committee.

Alexis arrived at the base with the shotgun, and "may have gained access to a handgun once inside the facility and after he began shooting," Parlave said. Investigators believe he may have shot and killed a security guard and taken his weapon. Contrary to previous accounts that were widely reported Monday, Parlave said Alexis had not used an AR-15 semiautomatic rifle.

Cathy Lanier, chief of Washington's Metropolitan Police Department, said police officers were at the Navy Yard within two minutes of the first calls for help. Within four or five minutes, she said, seven police units were on the scene, trying to determine where the gunfire was coming from.

"There were different buildings, different calls and different building numbers," she said. After a short time, she said, "we had units outside the building where the shooter was, and they could hear another round of gunfire. They entered immediately and two of them started giving lookouts and passing information along.

"There were multiple engagements with the suspect with multiple agencies before the final shots were fired," and this "saved numerous lives," Lanier said.

Parlave said the FBI believed Alexis acted alone. Ronald C. Machen Jr., U.S. attorney in Washington, said federal prosecutors nonetheless were investigating whether others assisted him in any way, even inadvertently. "We're not going to stop until we get answers to these questions," he said.

Melinda Downs, 44, a close friend from Alexis' days in Fort Worth, said she wished someone had helped him in other ways. Although he stayed with her and her husband during part of August, Downs said she had no clue that he had been hearing voices while in Rhode Island. She said he did not mention being followed or frequently changing hotels, nor his previous arrests for firing a gun through the ceiling of his apartment in Fort Worth and shooting the tires of a person's car in Seattle.

But, Downs said, she wished police had done more when he called them in distress.

"Did they try to get him some kind of help?" she wondered. "Take him to a counseling center?"

"When someone calls in with this type of mental instability, and you do nothing and yet he has access to all these weapons — the police who took that call have some kind of accountability," she said. "Why didn't they get him help?"

Serrano and Cloud reported from Washington and Hennessy-Fiske from Fort Worth. Times staff writers Tina Susman in New York and Richard Winton in Los Angeles contributed to this report.