US Defense Secretary Chuck Hagel acknowledged Wednesday authorities missed some "red flags" that might have averted a deadly shooting rampage at the Washington Navy Yard, vowing to close any gaps in security.

Hagel made the admission in the wake of Monday's massacre that left 13 dead, including the shooter, as details emerged of the troubled former sailor turned contractor who gunned down civilian workers at a naval installation in the heart of Washington.

"Obviously, when you go back in hindsight and look at all this, there were some red flags, of course there were," Hagel told a news conference.

"And should we have picked them up? Why didn't we? How could we have? All those questions need to be answered."

Reserving judgment as to who might bear the blame, Hagel unveiled details of a sweeping review of security at all military bases, which will include a look at how passes are issued to contractors.

The security clearances issued by the government are under intense scrutiny as the gunman, 34-year-old Aaron Alexis, had a valid pass that enabled him to enter the sprawling Navy Yard.

Alexis had a security clearance despite a record of misconduct in the Navy and run-ins with the law, including two shooting incidents and a Rhode Island police report showing he had severe delusions.

The ten-year security clearance, which was granted during his stint as a sailor from 2007-2011, remained valid once he left the service under an honorable discharge, according to the Navy.

Navy officials said none of his behavior during his time as a naval reservist would have disqualified him for a security clearance, as he had not been convicted in a military or civilian court for a serious crime and his offenses were not out of the ordinary.

In 2004, before he joined the Navy, Alexis shot up the tires of a car as he believed the owner had mocked him.

And in one incident in Texas, Alexis shot a bullet through his apartment ceiling, reportedly terrifying the woman who lived above him. But he told police it was an accident while he was cleaning his gun and he was not charged for any crime.

Only a month before the Navy Yard shooting, Alexis appeared to display the symptoms of paranoid schizophrenia, reportedly telling police in Newport, Rhode Island that he heard voices speaking to him "through the wall, flooring and ceiling."

The episode was relayed to military police at a base where Alexis was staying, but there was no legal cause to take action and as he was a contractor, no commanding officer to inform, officials said.

Hagel vowed to correct any flaws in security exposed by Monday's massacre.

"Where there are gaps, we will close them. Where there are inadequacies, we will address them. And where there are failures, we will correct them.

"We owe the victims, their families, and all our people nothing less," Hagel said.

Asked about Alexis' ten-year pass, Hagel said the duration of security clearances should be examined as well.

Speaking to CNN on Tuesday, Navy Rear Admiral John Kirby said Alexis had not had a "stellar" military record, but said there were no obvious danger signs.

Looking at "the offenses while he was in uniform, none of those give you an indication that he was capable of this sort of brutal, vicious violence," Kirby said.

The gunman's mother, Cathleen Alexis, issued an apology to victims' families and said she could not explain why her son had gone on a shooting spree.

"I don't know why he did what he did, and I'm never going to be able to ask him why," she said in a statement.

In Congress, lawmakers called for a review of security clearances while others renewed demands for stricter gun control measures.

House minority leader Nancy Pelosi, joined by family members of victims from the 2012 mass murder at the Sandy Hook Elementary School in Newtown, Connecticut and other mass shootings, urged the adoption of legislation that would require universal background checks for gun purchases.

"Let's take a vote," Pelosi said.

The Navy Yard reopened on Wednesday as the White House announced a memorial service for the victims scheduled for Sunday, which will be attended by President Barack Obama.

The president "will want to mourn the loss of these innocent victims and share in the nation's pain in the aftermath of another senseless mass shooting," White House spokesman Jay Carney said.