INDIANAPOLIS -- A gunman killed at least eight people at a FedEx facility in the midwestern U.S. city of Indianapolis before turning the gun on himself in the latest in a string of mass shootings in the country, authorities said Friday.

The incident came a week after President Joe Biden branded US gun violence an "epidemic" and an "international embarrassment" as he waded into the tense debate over gun control, a sensitive political issue in the United States.

After this latest killing he again ordered flags flown at half staff at the White House and other public buildings.

The gunman responsible for the overnight shooting was not immediately identified and it was not known if he was an employee at the FedEx facility near the airport in the state capital of Indiana, deputy police chief Craig McCartt told reporters.

"He got out of his car and pretty quickly started some random shooting outside the facility. There was no confrontation with anyone that was there. There was no disturbance. There was no argument. He just appeared to randomly start shooting and that began in the parking lot and then he did go into the building... for a brief period of time before he took his own life," McCartt said.

He said the shooting only lasted "a couple of minutes" and it appeared the attacker "took his life very shortly before officers actually entered the facility."

McCartt said the gunman was armed "with a rifle of some sort."

"This is a devastating day and words are hard to describe the emotion we feel," said Fedex chairman Frederick Smith in a letter to employees, adding that the company was working with law enforcement.

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 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."

Nervous relatives gathered at a hotel Friday morning near the plant for news of loved ones who work there and were not allowed to use their cell phones on the floor, news media said. 

Police spokeswoman Genae Cook told reporters that officers were called to an "active shooter incident" at around 11:00 pm (0300 GMT).

"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.

The plant is reported to employ more than 4,000 people.

Crime scene investigators walk through the parking lot of the mass shooting site at a FedEx facility in Indianapolis, Indiana, on April 16, 2021. Photo: AFP

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."

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 incident was the third mass shooting 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.

Biden this month announced six executive measures he said would help stem the gun violence crisis.

The move was immediately attacked by Republicans, with the party's senior leader in the House of Representatives, Kevin McCarthy, warning of "unconstitutional overreach."

"Gun violence is an epidemic in America. But we should not accept it. We must act," Biden said in a statement Friday after the new shooting.

"Too many Americans are dying every single day from gun violence. It stains our character and pierces the very soul of our nation," Biden said. 

"We can, and must, do more to act and to save lives."