A US Navy medic shot and wounded two sailors at a military facility on Tuesday, then fled to a nearby Army base where security forces shot and killed him.
Authorities said they had yet to determine what drove 38-year-old Fantahun Girma Woldesenbet to open fire at the facility, located in an office park in Frederick, Maryland.
"We're still trying to sort through stacks of paper ... to figure out exactly what the motive would be," Frederick Police Lieutenant Andrew Alcorn said.
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Mr Woldesenbet shot the sailors with a rifle inside the facility at the Riverside Tech Park on Tuesday morning, causing people inside to flee, Frederick Police Chief Jason Lando said.
Mr Woldesenbet, a Navy medic assigned to Fort Detrick but who lived in town, then drove to the base, where gate guards who had been given advance notice told him to pull over for a search, Brigadier General Michael J. Talley said.
But Mr Woldesenbet immediately sped off, making it into the installation before he was stopped at a parking lot by the base's police force.
When he pulled out a weapon, the police shot and killed him, Brigadier General Talley said.
The two sailors, who Brigadier General Talley said were assigned to Fort Detrick, were airlifted to a hospital.
Police said one victim is in critical but stable condition, and the other is in serious condition but expected to be released on Wednesday.
Brigadier General Talley said investigators will determine as much as they can, including why the suspect went back to the base.
"[I] don't know his mental status at the time, and we're certainly going to find all that out," he said.
The brigadier general said the facility where the shooting took place was not under his command.
He declined to identify the facility more specifically or describe the work that was done there.
Fort Detrick is home to the military's flagship biological defence laboratory and several federal civilian biodefence labs.
About 10,000 military personnel and civilians work on the base, which encompasses about 1300 acres in the city of Frederick.
The base is a huge economic driver in the region, drawing scientists, military personnel and their families.
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Frederick Mayor Michael O'Connor noted various defence contractors are located near Fort Detrick and that it wouldn't be unusual for a member of the military to be off base and working with a private firm that does business with the US government.
"When these incidents happen in other places, you're always grateful that it's not your community," Mr O'Connor added.
"But you always know, perhaps in the back of your mind, that that's just luck — that there isn't any reason why it couldn't happen here. And today it did."
By early afternoon, the Nallin Farm gate at Fort Detrick through which the shooter entered remained closed and two officers were standing by.
Police cordoned off Mr Woldesenbet's garden-style apartment building in Frederick City, a few miles from the site of the shooting.
A neighbor, Ava Target, said she knew Mr Woldesenbet only by sight, and that he lived on the top floor of the apartment complex with a wife and two kids.
She wasn't aware of any problems.
Another neighbour, Rachel Tucker, said she saw police escort Mr Woldesenbet's wife and two young children from the apartment early on Tuesday afternoon.
She said she believed the family had lived in the apartment for about a year and she never noticed anything out of the ordinary.
Liuetenant Alcorn said the crime scene unit had recovered multiple items from Mr Woldesenbet's apartment, but he declined to categorise them.
He said Mr Woldesenbet's wife had been at the apartment earlier on Tuesday and that police brought her in for questioning.
Mark Nelson, a firefighter who lives in a row of townhomes across the street from the base, said he heard the base blast warning sirens on Tuesday morning.
"I heard, I don't know what they call it, but they were like air raid sirens, and I knew something was going on," Mr Nelson said.
Mr Lando called the shootings "very tragic".
"It's happening too frequently," he said.
"Every time we turn on the TV we're seeing something like this happening. And now it's happening in our backyards."
