A former FedEx employee fatally shot eight people and wounded several others at the delivery company's facility in the Midwestern US city of Indianapolis before killing himself, authorities have said.
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 firearms control.
After this latest tragedy he again ordered flags flown at half staff at the White House and other public buildings.
Police in Indiana's capital city identified the gunman as 19-year-old Brandon Hole.
"FedEx officials have confirmed that Mr Hole was a former employee at the facility, and he was last employed in 2020", deputy police chief Craig McCartt told reporters, adding that authorities were gathering evidence in multiple locations as they sought to ascertain a motive.
At least 100 people were in the facility at the time of the overnight shooting, which occurred while many people were changing shifts or on their dinner break, Mr McCartt said.
It was not immediately clear whether Mr Hole had been fired or left his job voluntarily.
"He got out of his car and pretty quickly started some random shooting outside the facility", Mr McCartt said
"There was no confrontation with anyone that was there. There was no disturbance. There was no argument."
He said the shooting "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."
Mr McCartt said the gunman was armed with a rifle.
"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 shipping center 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.
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 mobile phones on the floor, news media said.
Police spokeswoman Genae Cook told reporters that officers were called to an "active shooter incident" at around 11pm Thursday (0300 GMT Friday).
"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," Ms Cook said.
The plant is reported to employ more than 4,000 people.
The Indianapolis carnage follows a spate of mass shootings across the US in recent weeks.
Late last month four people, including a child, were shot dead in an office building in southern California.
On 22 March, 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, while 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 US gun regulation is politically fraught.
Joe 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," Mr 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."
Senate Majority Leader Chuck Schumer said he will take action.
"I will bring gun violence prevention legislation to the Senate floor," he tweeted. "We must break this cycle of suffering in America."