A gunman shot eight people dead at a FedEx facility in Indianapolis before killing himself, police have said.
Officers rushed to the site on Mirabel Road in the US city at around 11pm on Thursday after reports of a shooting.
Police have confirmed that eight people have been killed in the horror incident, not including the shooter, with many others injured.
An earlier statement said that Indianapolis Metropolitan Police Department officers had dealt "with an active shooter incident".
Witnesses have described seeing a man with an "automatic rifle" firing at people.
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An investigation into the tragedy has been opened, with no motive suggested so far by authorities.
Spokeswoman Genae Cook said: "Today it is very heartbreaking.
"The officers responded, they came in, they went in and they did their job.
"Preliminary information at this time is that the shooter has taken his own life.
"We don't feel there is an active threat to the community at this time."
Ms Cook said that four people were transported to hospitals, one in critical condition and three others with various other injuries.
Two others were treated at the facility by medical staff and released and that "multiple" others had walked to hospitals, she added.
Family members of FedEx workers attempting to visit the scene have been told to go to a hotel, police said.
André Carson, congressman for Indiana's seventh district, has tweeted his condolences following the shooting.
He said: "I am heartbroken by the mass shooting at the FedEx facility here in Indianapolis and praying for all affected by this tragedy.
"I am communicating with local authorities to get all details of the attack and my office stands ready to help everyone affected any way we can."
Jeremiah Miller said he had finished a shift at the facility, which employs more than 4,500 people, and was about to work a second consecutive shift when he and a colleague heard gunfire.
He said: "This made me stand up and actually look out the entrance door, and I saw a man with a submachine gun of some sort, an automatic rifle, and he was firing in the open.
"I immediately ducked down and got scared and my friend’s mother, she came in and told us to get inside the car."
His colleague Timothy Boillat was reported to have said: "I didn’t exactly see a person get shot but, after hearing the shooting, I did see a body on the floor behind a vehicle.
"Luckily, I was far enough away where he (the shooter) didn’t notice me or see me. So, thank God for that."
Images from the scene appear to show a large group of employees standing outside the building with a heavy police presence in place.
In a statement, FedEx said: "We are aware of the tragic shooting at our FedEx Ground facility near the Indianapolis airport.
"Safety is our top priority, and our thoughts are with all those who are affected.
"We are working to gather more information and are cooperating with investigating authorities."
The incident comes after a string of mass shootings left scores of people dead in the US in recent weeks, including at Boulder, Colorado and in and around Atlanta.
