ST. LOUIS — An employee of a St. Louis manufacturing plant walked in with an assault rifle and a handgun this morning and opened fire, killing at least three people and wounding five others, authorities said.

Several hours after the shooting, police were still inside Swiss-based ABB Group’s plant, going room to room in a search for both the gunman and additional victims, police Capt. Sam Dotson said. Authorities had not confirmed whether the shooter was among the eight.

Fire Department spokesman Bob Keuss identified the suspected shooter as Timothy Hendron of Webster Groves. Dotson said Hendron is an employee of the plant.

In 2006, Hendron and other ABB workers sued the company over retirement losses. The federal lawsuit accused ABB and its pension-review committee of causing their 401(k) accounts to include investment options with “unreasonable and excessive” — and undisclosed — fees and expenses. The suit went to trial Tuesday in Kansas City.

The shooting occurred around 6:30 a.m. during a shift change, and 40 to 50 people were likely in the plant at the time, Dotson said. As shots began to ring out, employees scurried to find safety, authorities said.

“Many of them sought safety on the roof, in boilers and broom closets,” Dotson said.

Names of the victims were not immediately released. Police said three of the injured were in critical condition and two were in fair condition.

Fire Chief Dennis Jenkerson said firefighters and paramedics were standing by in case there were additional injuries, either from the shooting itself or from seeking refuge on the roof in bitter cold. The wind chill in St. Louis dipped below zero.

“It’s cold, and shock sets in, hypothermia,” Jenkerson said. “It doesn’t sound good.”

Dozens of emergency vehicles circled the sprawling plant on a day made more chaotic by several inches of snow that snarled traffic in the St. Louis region.

ABB Group makes power transmission and industrial automation equipment. The company manufactures transformers at the St. Louis site, which employs about 270 people. The company has operations in roughly 100 countries, employing about 120,000 people. Last October, ABB reported third-quarter earnings of more than $1 billion.

Thomas Schmidt, an ABB corporate spokesman in Zurich, Switzerland, said in statement today: “This is obviously a very serious situation and we are working to gather more information as it becomes available. The welfare of our employees is of utmost importance to us.”

Word of Hendron’s alleged involvement in the shooting stunned his neighbors in Webster Groves, an upscale St. Louis suburb. Many neighbors described Hendron as an amicable family man who kept a well-manicured home for his wife and small boy.

“I couldn’t ask for a better neighbor. We never had any problems with him,” said Glennon Meyer, a 71-year-old retiree who credits Hendron with friendly gestures ranging from raking Meyer’s leaves to bringing over a chocolate cake last Christmas.

A few years ago, Meyer said, Hendron mentioned something in passing about having problems on the job. Hendron didn’t elaborate.

“Gee, I’ve talked to Tim many times, and he never exhibited any mental aberration,” Meyer said.

Ron Hawkins, who lives across the street from Hendron’s split-level home, echoed that.

“He seems like a really nice guy,” said Hawkins, 72. “I know nothing negative about he and the family.”