Three other workers were injured in the Monday afternoon accident that may have sent thousands of gallons of sewage into the river. State officials started an investigation into the cause of the explosion.

The sewage treatment plant continued to operate Tuesday without the damaged tank, officials said.

"We do not see the public being at any risk from the incident itself or remaining operations of the treatment plant," said Fire Chief Bobby Williams.

The missing man was identified as Mike Cmos, a 24-year employee of the city who was a mechanic at the plant, Williams said.

Dan Evans, 52, a mechanic who has worked for the city 28 years, underwent surgery at Sacred Heart Medical Center Monday night and remains in the hospital, Williams said.

Employees Larry Michaels and Tim Pelton suffered minor injuries and were not hospitalized, Williams said.

The ages of Cmos, Michaels and Pelton were not immediately available.

A search for several hours along the river Monday turned up no sign of Cmos, and authorities believed he was in the tank at the Wastewater Treatment Plant northwest of the city.

Williams said pumping had lowered the 40-foot-deep tank's smelly contents by about 18 feet overnight and would continue.

The four workers were either on or near the tank to check on an unusual pressure buildup when a "relatively small hole" blew open near the top of the tank at 3 p.m. Monday, Williams told a news briefing.

That loosened the tank's lid, which fell into the tank and caused up to 200,000 gallons of sludge and wastewater to spill onto the parking lot, river bank and into the water, Williams said.

"I heard a big boom, followed by two smaller noises," said Betty Gaither, who lives on a bluff directly above the tank that burst. "It felt like a small earthquake.

"I walked outside and what we saw was just terrible," she told the Spokesman-Review. "It was like a lake of sewage several inches deep flowing down the bank. The river - clear across - was brown."

No evacuations were ordered and there was no danger of additional ruptures, Williams said.

The cause of the pressure buildup was under investigation. The state Department of Labor and Industries, which investigates industrial workplace accidents, was interviewing plant employees, the agency's Spokane compliance officer Dana Aga said.

The Spokane Regional Health District warned people downstream to avoid withdrawing drinking water from the river for themselves, livestock or crop irrigation for at least three days.

Washington Department of Ecology spokeswoman Jani Gilbert said there are no municipal drinking water withdrawal points downstream from the plant. City employees will be testing the river water for bacterial contamination, she added.

The plant is on the Spokane River near Riverside State Park, several miles northwest of the downtown area. The tank is one of three "digester" tanks at the complex, which treats 44 million gallons of sewage a day.

Digesters break down sewage sludge before the liquid is removed, leaving a sludge that generally is sold to farmers as fertilizer.