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 a military contractor gunned down 12 people and wounded eight others.
The installation is closed Tuesday to all but essential personnel. Investigators 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 the gunman, Aaron Alexis -- a former Navy reservist 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.
Alexis 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.
In addition to the dead gunman, whom authorities identified earlier, police late Monday released the names and ages of seven of the 12 people killed in the shooting. None of the seven was military personnel. 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.
Eight people were injured in the shooting, Washington Mayor Vincent Gray told reporters Monday night. Three of those were injured by gunfire, and the others had other types of injuries, such as bruises and chest pain. Earlier Monday evening, Navy Vice Adm. William D. French said 14 people were injured.
Washington police are confident that only one person was involved in Monday morning's shooting at the Washington Navy Yard, and they are lifting a shelter-in-place order for residents who live nearby, Police Chief Cathy Lanier said Monday night.
Tell us about the shooter
Authorities are still searching for more information about Alexis, 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."
Even as the FBI ruled out any other shooters in the rampage at the headquarters for Naval Sea Systems Command, Metropolitan Police were trying to track down at least one person to determine whether that individual had any involvement.
"We'll continue to seek information about what the motive is. We don't have any reason at this stage to suspect terrorism," Gray told reporters, "but certainly it has not been ruled out."
The other possible suspect was described by police as a man, between 40 and 50, wearing an "olive drab-colored" military-style uniform.
"We still don't know all the facts. But we do know that several people have been shot and some have been killed," President Barack Obama said Monday afternoon. "So we are confronting yet another mass shooting. And today it happened on a military installation in our nation's capital."
Obama called the shooting a "cowardly act" that targeted military and civilians serving their country.
"They know the dangers of serving abroad," he said, "but today they faced the unimaginable violence that we wouldn't have expected here at home."
Witness: People pushed their way out of building
The violence began at 8:20 a.m., when several shots were fired inside 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. 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."
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 D.C. 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.
Details about gunman