At least eight people were killed in a shooting at a delivery company by a gunman who is believed to have then turned the gun on himself, police in the US city of Indianapolis said.
The victims all died at a Fedex facility where the shooting took place late Thursday, police spokeswoman Genae Cook told a news conference, adding several other people had been taken to hospital.
Four people with gunshot wounds were transported by ambulance, including one in critical condition, police said. Three were transported with other injuries, while two were treated at the scene and then released.
One man who was working a twilight shift at the plant near the city's international airport told local broadcaster WISH-TV he saw the gunman start shooting and heard more than 10 shots.
"I saw a man with a sub-machine gun of some sort, an automatic rifle, and he was firing in the open. I immediately ducked down and got scared," Jeremiah Miller said.
"My friend's mother, she came in and told us to get inside the car. What we've been doing [since] is telling everyone, our co-workers not to go to work today."
Cook told reporters that officers were called to an "active shooter incident" at around 11:00 pm (0300 GMT), adding they believed the gunman -- who has not been identified -- died by suicide.
"The officers responded, they came in, they went in and they did their job. A lot of them are trying to face this because this is a sight that no one should ever have to see," Cook said.
Authorities judged there was no longer an immediate threat to public safety, Cook told reporters, and police were still gathering information to identify the victims.
"We have located eight people at the scene with injuries consistent to gunshot wounds. Those eight were pronounced deceased at the scene," Cook said.
"This is a tragedy."
A Fedex spokeswoman confirmed to AFP that the company's facility was the scene of the shooting, and said the firm was cooperating with authorities.
"We are aware of the tragic shooting at our ground facility near the Indianapolis airport," the company said in a statement.
"Safety is our top priority, and our thoughts are with all those who are affected."
The plant is reported to employ more than 4,000 people.
Timothy Boillat, another employee, told WISH-TV that he saw around 30 police cars arriving at the scene as he witnessed the shooting unfold.
"After hearing the shootings, I did see a body on the floor," he said.
"Luckily, I was far enough away to where he [the shooter] didn't see me."
- Spate of shootings -
The incident in Indianapolis follows a spate of mass shootings across the US in recent weeks.
At the end of last month, four people, including a child, were shot dead in an office building in southern California.
On March 22, 10 people were killed in a shooting at a grocery store in Boulder, Colorado.
That came less than a week after a man shot and killed eight people, including six women of Asian descent, at spas in Atlanta, Georgia.
Thursday's shooting was the third in Indianapolis this year. In January, five people including a pregnant woman were killed; three adults and a child were killed in March.
Nearly 40,000 people in the United States die each year from guns, more than half of those being suicides.
The issue of gun regulation in the United States is politically fraught.
President Joe Biden this month announced six executive measures he said would help stem the gun violence crisis.
"It's an international embarrassment," Biden told Congress members and gun control activists at a White House ceremony.
"Enough prayers," the Democrat said. "Time for some action."
The move was immediately attacked by Republicans, with the party's senior leader in the House of Representatives, Kevin McCarthy, warning of "unconstitutional overreach."