Shaun Cooper, who works at supplier Fred Tapping, across the road, said two of his colleagues heard five shots fired.
"The cops were there within three or four minutes," he said.
Mr Cooper said he saw police remove one body from the Inline building, while another two people were dragged out.
One person was helped to a waiting ambulance while the other was able to get in unaided.
"You couldn't see any visible signs of blood," he said of the two injured men.
His office is directly across the road from Inline Signs, and staff could see the police SWAT team surround the building from their top floor.
An employee of Konecranes, which is opposite Inline, said police had told them to stay inside their building while the operation was under way across the street.
"We heard five shots. There have been plenty of police and ambulances," said the woman, who wanted to remain anonymous.
"We have been told to stay indoors and away from the windows."
A worker at another business on Heald Road said staff had also been told to stay indoors.
"We are in lockdown so no one can leave the building or come in," she said.
"We are just sitting quiet, waiting for it all to quieten down out there."
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