WASHINGTON (CNN) -- The normal buzz of the Washington Navy Yard's 3,000 workers will be replaced by the meticulous work of forensics teams, looking for answers after military contractor Alex Alexis gunned down 12 people and wounded eight others.
Aside from investigators, only a few essential employees will be on the base Tuesday. Authorities have questions to ask, measurements to take and information to sift through.
Mid-morning, Defense Secretary Chuck Hagel and other Department of Defense leaders will lay a wreath at the Navy Memorial plaza to honor the victims of Monday's shooting.
Dead is Alexis, a former sailor with a "pattern of misconduct," and 12 others, a mix of civilian workers and military contractors.
Authorities said Alexis was killed after an encounter with security. They gave no other details.
He began at the Navy Yard last week, but worked at multiple Navy offices over the summer, said employer Thomas Hoshko, CEO of The Experts, an HP subcontractor. Hoshko said there were no reports of problems with Alexis at the other Navy offices.
Two law enforcement sources say Alexis recently made contact with two Veterans Affairs hospitals. His contact is initially believed to be for psychological issues, the sources said.
In addition to the 12 deaths, eight people were injured, Washington Mayor Vincent Gray told reporters Monday night. Gunfire injured three, and the other victims suffered bruises, chest pain and other injuries, he said.
Earlier Monday evening, Navy Vice Adm. William D. French said 14 people were injured.
Despite earlier reports suggesting as many as two other people may have been involved, Washington police are confident that only one person was involved in Monday morning's shooting at the Washington Navy Yard, police Chief Cathy Lanier said Monday night.
Metropolitan Police were trying to track down at least one person -- described by police as a man, between 40 and 50, wearing an "olive drab-colored" military-style uniform -- to determine whether that individual had any involvement.
Seeking information about the shooter
What motivated Alexis to kill was a mystery Tuesday Terrorism hasn't been ruled out but seems unlikely, Gray told reporters.
Authorities were searching for more information about him, and they're asking the public for help.
"No piece of information is too small," said Valerie Parlave, assistant director in charge of the Washington FBI Field Office. "We are looking to learn everything we can about his recent movements, his contacts and associates."
A friend and former housemate, Kristi Suthamtewakul, told CNN's "New Day" that she had noticed personality changes in Alexis over the last few months, but nothing indicating the potential for such violence.
"Aaron was a very polite, very friendly man," she said.
Among other problems, he had been frustrated about pay and benefits issues after a one-month contracting stint in Japan last year, Suthamtewakul said.
"He got back and he felt very slighted about his benefits at the time," she said. "Financial issues. He wasn't getting paid on time, he wasn't getting paid what he was supposed to be getting paid."
"That's when I first started hearing statements about how he wanted to move out of America," Suthamtewakul said. "He was very frustrated with the government and how as a veteran he didn't feel like he was getting treated right or fairly."
Another friend, Texas resident Michael Ritrovato, said Alexis had recently been frustrated with his employer over pay.
But Ritrovato said his friend never showed signs of aggressiveness or violence, though he played a lot of shooting video games online.
"It's incredible that this is all happening, because he was a very good-natured guy," Ritrovato said. "It seemed like he wanted to get more out of life."
In Seattle, police said they arrested Alexis in 2004 for shooting out the tires of another man's vehicle in what Alexis later told detectives was an anger-fueled "blackout."
He was also arrested in DeKalb County, Georgia, in 2008 on a disorderly conduct charge, police there said Tuesday.
Before working as a military contractor, Alexis had served in the Navy as a petty officer working on electrical systems.
He had a "pattern of misconduct" while in the Navy, but was honorably discharged, two Navy officials told CNN.
Alexis was in the Navy's Ready Reserve, Navy Secretary Ray Mabus told CNN
Ready Reserve status is a designation for former military members who don't actively serve in a Reserve unit but who can be called up if the military needs them.
Witness: People pushed their way out of building
Monday's violence began at 8:20 a.m., when several shots broke the workday calm of the southeast Washington facility.
D.C. Metropolitan Police officials told CNN that Alexis drove onto the installation and parked before walking a short distance to Building 197, where the killings occurred.
Alexis had an active ID and entered the base legally, a federal law enforcement official said.
Once inside, according to an official, Alexis made his way to an overlook above the atrium and opened fire.
Two witnesses told CNN affiliate WJLA-TV that they heard a fire alarm go off in the building where they worked, then saw a man with a rifle down the hallway as they left the building.
"He aimed the gun and fired our way," Todd Brundidge told WJLA.
People frantically ran down stairs to get out of the building, Brundidge said.
"They were pushing. They were shoving. People were falling down," he told WJLA. "As we came outside, people were climbing the wall trying to get over the wall to get out. .... It was just crazy."
Police late Monday released the names and ages of seven of the 12 people killed in the shooting.
None of the seven was in the military.
They are Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61.
The names of the other five will be released once their families have been notified.
The injured included a Washington police officer who has been hospitalized and a base security guard officer, said Metropolitan Police Department spokeswoman Saray Leon.
Three people, including the Washington police officer, were admitted to MedStar Washington Hospital Center with multiple gunshot wounds. They are expected to survive, chief medical officer Janis Orlowski told reporters.
One person was pronounced dead at George Washington University Hospital, said Dr. Babak Sarani, chief of trauma and acute care there.
Alexis died after the killings in an encounter with security. The FBI said it identified him using fingerprints and his identification card.
Authorities have recovered three weapons from the scene of the mass shooting, including a shotgun that investigators believe Alexis brought into the compound, federal law enforcement sources with detailed knowledge of the investigation told CNN on Tuesday. The other two weapons -- handguns -- may have been taken from guards, the sources say.
Earlier information that Alexis may have used a semiautomatic AR-15 rifle may have been incorrect, the sources said.
Investigators believe Alexis rented an AR-15 but had returned it before Monday's shootings, the sources said. Authorities are still investigating how many weapons Aaron had access to, the sources said.
Alexis probably carefully planned the killings, said Chris Voss, a former FBI agent.
"The shooter rehearsed this in his mind, and he probably walked through it a few times physically ahead of time," he told CNN's Chris Cuomo.
"He wanted to make sure he did this," Voss said. "So he came in very focused, made sure he wasn't distracted, he probably did exactly as he had rehearsed previously."
Kirtk Lippold, a retired U.S. Navy commander and military policy expert at Phillps Stutts, has been inside Building 197 and thinks Alexis must have known he would have to fight his way inside.
"I think he actually fought his way in and was killing people on his way to the third floor where he started that rampage," said Lippold, who was commander of the USS Cole when that ship was attacked by terrorists in Yemen in 2000.
Navy Yard history
The military installation will be closed to all but emergency personnel and traffic Tuesday, the Naval District of Washington's Facebook page said.
The base is headquarters for Naval Sea Systems Command, which "builds, buys and maintains the Navy's ships and submarines and their combat systems," according to the Navy.
Originally envisioned as facility to build and outfit ships on the Anacostia River, it serviced some of the Navy's most famous early vessels, including the USS Constitution.
Burned during the War of 1812, the Navy Yard was transformed into a center for weapons and technological development.
The facility was the world's largest ordnance plant during World War II, but its military role steadily diminished during the Cold War era.
Today, the Navy Yard includes the headquarters of Naval District Washington and a naval museum.
CNN's Barbara Starr reported from Washington, and CNN's Catherine E. Shoichet reported from Atlanta. CNN's Chris Cuomo, John King, Deborah Feyerick, Evan Perez, Tom Cohen, Dan Merica, Larry Shaughnessy, Brian Todd, Alan Silverleib, Susan Candiotti, Joe Johns, Eliott C. McLaughlin, Joe Sterling, Paul Courson and Ed Lavandera contributed to this report.