Navy Yard shooter Aaron Alexis practiced shooting at a gun range two days before killing a dozen people, according to the shooting range.
Authorities have also determined that he was armed with a shotgun and at least one handgun he took from a guard he shot.
Alexis visited Sharpshooters Small Arms Range in Lorton, Va., on Saturday, J. Michael Slocum, an attorney for the store, said today. Alexis rented a rifle and bought ammunition, which he used at the practice range.
(Slocum said in his original statement today that Alexis visited the gun range on Sunday, but later corrected himself and said it was on Saturday.)
Alexis then purchased a Remington 870 shotgun and about two boxes, or 24 shells, of ammunition, the store said. Alexis provided all of his personal information to the vendor and was approved by the system, in accordance with federal law.
"After the terrible and tragic events at the Navy Yard, the Sharpshooters was visited by federal law enforcement authorities, who reviewed the range's records, including video and other materials," Slocum said in a statement. "So far, as is known Mr. Alexis visited the range only once, and he has had no other contact with the Range, so far as is known."
Alexis' practice on the eve of the massacre adds eerie credibility to a statement by Metropolitan Police Chief Cathy Lanier today who said described Alexis today as "a gunman who was determined to kill as many people as possible."
The chief praised the cops in who went in after him.
"There is no doubt in my mind they saved numerous lives," Lanier said.
On Monday, Lanier described said there had been several engagements between Alexis and cops before he was finally shot dead.
Federal authorities are combing through Alexis' laptop computer hoping to find a clue to his rage that left 12 people dead, law enforcement sources told ABC News.
Authorities are also going through Alexis' hotel room hunting for a reason why he massacred so many people.
Valerie Parlave, head of the FBI's field office in Washington, said officials believe Alexis entered Building 197 of the Naval Sea Systems Command Headquarters with a shotgun.
"We do not have information at this time that he had an AR-15 in his possession," Parlave said, referring to early news reports that was the weapon he used.
After entering the building, Alexis went to a men's bathroom and then to the fourth floor of the building, where he started shooting, federal and local law enforcement officials confirmed.
He proceeded to the third floor and fired shots into the cafeteria before going down a stairwell and toward the building's front entrance where he shot at security guards, officials said. He picked up at least one of the guard's handguns.
Lanier suggested a fierce gunfight between Alexis and the "active shooter teams" who entered the building. She said the shooting lasted more than 30 minutes.
Alexis had "legitimate access to the Navy Yard" and used a valid pass to gain entry to the building, Parlave said.
A Defense Department official said that Alexis had "secret" security clearance granted in March 2008 and that level of security clearance lasts for 10 years. Secret is the lowest of the three security clearances a person can get.
As is customary, when he left the Navy in January 2011 with an honorable discharge he was able to carry over the clearance to the civilian world.
Alexis worked for Hewlett-Packard as an IT subcontractor for the Navy, the company said. He was an employee of a company called "The Experts" that refreshed equipment used on the Navy Marine Corps Intranet network.
"We can confirm that the suspect had been employed by The Experts for approximately six months over the last year, during which time we enlisted a service to perform two background checks and we confirmed twice through the Department of Defense his Secret government clearance," The Experts said in a statement today.
"The latest background check and security clearance confirmation were in late June of 2013 and revealed no issues other than one minor traffic violation," the company said.
The shooting was first reported at 8:15 a.m. Metropolitan Police Chief Cathy Lanier said officers were on the scene within two to three minutes of the first call.
She said that internal security had already engaged the shooter and victims were already down. An active shooter team was on the site within seven minutes and Alexis was killed in a gunfight with authorities.
The FBI is asking that anyone with information on Alexis call 1-800-CALL-FBI.