Five people were confirmed dead after a gunman used a laser-sighted pistol in a shooting at a factory in Illinois
Those injured included five police officers who responded to the attack.
Donald Trump was being kept informed about the incident, the White House said.
The gunman was identified by police as Gary Martin, 45, who worked at the valve factory in the city of Aurora.
John Probst, a worker who was in the factory, said the gunman had arrived for his regular shift, working in assembly, and opened fire several hours later.
Mr Probst told local television: “I saw a gunman running down the aisle with a pistol, I recognised him as a co-worker and he was shooting everybody.
“As soon as we heard shots, as soon as I saw the green laser thing, we took off out the back door.”
Mr Probst said the plant was “huge” and the man worked about 80 yards from him.