(CNN) Gary Martin opened fire with a Smith & Wesson on the day he was fired from the Henry Pratt Co. in Aurora, Illinois , killing five workers and wounding five police officers.

The victims included a human resources manager, a human resources intern and a plant manager at the valve manufacturing company where the gunman worked for 15 years.

"The only thing I know is that he was called in," Aurora Police Chief Kristen Ziman said. "Once again we can surmise that he was speculative about what was going to happen as evidenced by him arming himself with a firearm. ... I don't know exactly what was communicated to him."

Gary Martin is seen in this image taken from Facebook.

As details surrounding Friday's deadly rampage begin to emerge, the 90 minutes of terror at the Aurora warehouse highlight a disturbing trend: More than 2 million Americans each year report being victims of workplace violence, according to the federal government . Many more cases go unreported.

"The point of termination is perhaps the greatest opportunity for deadly workplace violence," said Kathleen Bonczyk, the founder and executive director of the Florida-based non-profit Workplace Violence Prevention Institute

It's not clear why Martin was being fired or whether he knew of his termination beforehand.

But Bonczyk said the firing of longer-term employees can involve the greatest risk.

"It's almost like a divorce from a family," she said. "Americans today tend to spend more time at work than they do even at home. They tend to have more lunches, dinners, meals, time spent with their coworkers. You're severing perhaps the most stable relationship that the employee may have."

There were 458 workplace homicides across the nation in 2017, with 351 of those being shootings, according to the most recent government figures available . A year earlier, there were 500 workplace homicides, including 394 fatal shootings.

"It's a horrifyingly familiar feeling because this public safety threat has cast a shadow over every aspect of American life," she said in a statement.

Giffords was shot in the head in January 2011 at a "Congress on Your Corner" event at a grocery store in Tucson, Arizona. She was one of 13 people wounded by Jared Lee Loughner, whose primary target was the now-former congresswoman. Six others were killed.

In Aurora, Martin shot several of the men at a meeting during which he was being fired, police said. At least one of the slain men was found on another level in the building.

He was legally barred from owning a gun because he had a 1995 felony aggravated assault conviction in another state, Ziman said. He also had six prior arrests, including one for domestic battery.

When the shooting started, Martin was in the meeting in an upstairs room at the warehouse. He brought the gun, apparently concealed, into the room, Ziman said.

"My understanding from witnesses is that he opened fire right after the termination," she said.

Several of the people in the meeting were killed -- and witnesses said he walked out of the room and continued to fire. Several people called 911.

At least one worker was found dead on a separate level from the meeting room, Ziman said.

Bonczyk said a prior conviction for aggravated assault is a major warning sign of potential workplace violence.

"The portrait that appears to be emerging is of a volatile person with a temper, in possession of a firearm that he was not supposed to have," she said.

Scott Hall, president and CEO of Mueller Water Products, the parent company of Henry Pratt, told reporters Saturday that Martin was being terminated for "a culmination of various workplace rule violations." He would not characterize the violations.

A background check at the time of his hiring 15 years ago did not show a felony conviction, Hall said.

"We're assessing if there is anything we could have done or could do in the future," Hall said, adding that he was certain there were some shortcomings that would need to be addressed.

Bonczyk said employers should look for warnings when first interviewing potential employees. Red flags include marital problems and domestic violence, disputes with coworkers, financial stress and drug and alcohol abuse. She said employee assistance programs can help identify potential problems.

"Are they having problems with the law? Are they having problems with other people? Do they have access to weapons? Are they posting photographs of their weapons online?"

It's not known whether Martin's coworkers spotted any red flags before he turned his gun -- and a laser sight -- on them. Officers found Martin in the rear of the warehouse. He opened fire and officers fired back, killing him, police said.

"The time to think that it can't happen here, that it couldn't happen to me, is long gone," Bonczyk said of violence in the workplace.

"We do have a growing epidemic. I hope that people will take this opportunity to learn from this tragedy and look at the shortcomings within their workplaces, because it's happening everywhere."