WASHINGTON, D.C. -- At least 13 people are dead after a shooting Monday morning in a heavily secured building at the D.C. Navy Yard, and authorities now say they have identified the gunman.

The shooting is one of the worst mass shootings on a U.S. military base in recent history. In November, 2009, 13 people were killed by a shooter at Fort Hood, Texas.

The gunman, 34-year-old Aaron Alexis of Fort Worth, Texas, is among the 12 dead. Officials said he recently began working as a civilian contractor.

The FBI, which now is leading the investigation, believes the gunman used the identification of a man who used to work at Navy Yard, reported Pete Williams of NBC News. The ID was found next to the gunman's body in Building 197, where most of the victims were shot on the third and fourth floors.

People with proper badges are allowed to enter Navy Yard without having their bags checked, Williams reported.

Multiple weapons

Survellance video shows the gunman entered the building at the Naval Sea Systems Command Headquarters, at 1336 Isaac Hull Ave., with a shotgun, News4's Jackie Bensen reported.

He shot the security guard in the head, killing him. The shooter then continued through the building, and seemed to target his victims, who were mostly on the third and fourth floors.

D.C.'s Metropolitan Police Department and several other law enforcement agencies responded, Bensen said. During that response, a MPD officer was shot in the leg.

The gunman was then shot by a FBI hostage response team, Bensen said.

According to what witnesses are telling investigators, by the time the shootings ended, the gunman was seen with a semiautomatic 9 mm pistol and an AR-15 assault rifle. Authorities are investigating whether the gunman took the security guard's service weapon - likely a 9 mm pistol - and hid in wait for the first responding D.C. police officers, who would be specially armed with AR-15s.

D.C. police had said earlier Monday that they were seeking two other "possible" gunmen. Investigators later ruled them out as suspects by watching video that tracked them through the building, and realizing that no harm resulted from their movements.

Williams noted that many people are authorized to carry firearms at Navy Yard.

Obama: 'They're patriots'

"We do know that several people have been shot, and some have been killed," President Obama said from the White House Monday. "So we are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital. It's a shooting that targeted military and civilian personnel.

"These are men and women going to work, doing their job protecting all of us," Obama said. They're patriots, and they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn't have expected here at home."

Though Metropolitan Police Chief Cathy Lanier stressed that the scene was contained, people near the base were asked to stay in their homes, and eight D.C. public schools and one private school nearby were locked down.

A large search, including from helicopters, continued into Monday afternoon. "It's a large piece of land with many buildings," Assistant D.C. Police Chief Peter Newsham told News4. "It's going to take a while to determine that it is safe."

Earlier Monday, tactical teams were seen entering at least two buildings on the Navy Base to search for the shooter, and News4's Mark Segraves reported that three helicopters were seen tightly circling part of the base.

Treating the victims

MedStar Washington Hospital Center said the MPD officer was among three victims that had been transported to that hospital. The officer was shot in the legs; one other victim was shot in the shoulder and one was shot in the head and hand.

All are alert and speaking at this time, and there is a "very good chance they will survive," said Dr. Janis Orlowski, chief medical officer at the hospital.

However, the hospital has been prepared to expect more victims.

George Washington University Hospital said they had also received a victim, a man in his 60s who had been shot in the temple. Despite efforts at CPR, he was prounounced dead at the hospital.

One victim was evacuated from the roof of a building, reported News4's Tony Tull. A U.S. Park Police helicopter hovering above the scene around 9:50 a.m. lowered a basket to a building and lifted what appeared to be a shooting victim from the roof. The helicopter came back to the scene just after 10 a.m. to retrieve another victim.

"I was in the cafeteria, just paid for my breakfast," Patricia Ward said. "I was waiting for my friend to pay for hers when we heard the gunshot. It was three gunshots straight in a row, pop pop pop, three seconds later, it was pop pop pop pop pop, so it was like about a total of seven gunshots, and we just started running."

Federal and local emergency personnelhad converged on the scene, and a shelter in place order has been issued for Navy Yard personnel, the Navy said.

Anyone with information about this incident should call 1-800-CALL-FBI.

Family members seeking information about loved ones should call 202-433-6151 or 202-433-9713.

The Navy Yard is located in southeast Washington on the Anacostia River. The Navy says 3,000 people work inside the building, including command staff, headquarters directorates and field activities. They coordinate the Naval Sea Systems Command, or NAVSEA, the largest of the Navy's five system commands.