Trouble at the Suncor Energy oil refinery north of Denver on Wednesday morning triggered a “vapor release alarm,” prompting operators to place one of their units into “safe mode” and announce they’ll launch a full investigation as soon as possible.

Suncor officials called it “an opacity event” in a statement emailed to local officials and posted on the company’s Facebook page. They said the company’s tests showed air quality in surrounding neighborhoods “is within acceptable levels.”

Children at two nearby schools were told to shelter in place after ash started falling on cars following the release, Commerce City police said.

Wednesday evening, Suncor issued a second statement, saying materials released included non-hazardous, clay-like “catalyst.” They advised hand-washing and offered free car washes to residents whose vehicles were covered with ash.

“Our first priority is the safety of our employees, contractors and neighbors. We will continue to work with stakeholders to address issues and concerns,” company officials said in a statement.

Air pollution control officials at the Colorado Department of Public Health and Environment said they were planning to review air-monitoring data collected by the state.

Smoke spewed out of stacks around 10:45 a.m. Wednesday, the result of what officials described as “an operational upset” at the refinery, located in Commerce City just north of Denver. This incident followed another “operational upset” around 6 a.m. Nov. 27, according to company statements.

Refinery workers were moved to a safe area and “community air monitoring is in place,” company officials said in their statement.

On Wednesday, two neighborhood schools — Alsup Elementary and Adams City Middle School — were put on a “shelter in place alert” after ash fell on cars, Commerce City police Chief Clint Nichols tweeted. South Adams Fire later “determined that no hazard exists,” Nichols wrote.

@SACFD200 has determined that no hazard exists. Ash potentially from an earlier Suncor burn off. https://t.co/pYXWHp4Aih — ChiefNichols (@CNichols004398) December 11, 2019

Suncor officials declined to discuss what happened, referring The Denver Post to their written statements.

“The appropriate local authorities and regulatory agencies have been notified and we are working closely with them,” Suncor officials wrote. “We will initiate a full investigation as soon as response activities allow, after which we will look to bring the unit back to safe and reliable operation.”

The catalyst material emitted from the refinery spread into neighborhoods and landed on vehicles, buildings and machinery. People who come into contact with it “should take standard hygiene practices such as washing hands and/or any affected clothing. Additionally, we recommend that any surface observed to be covered in catalyst be washed with water,” Suncor’s statement said.

“Individuals whose vehicles were in the affected area and impacted by catalyst may visit one of the below car wash locations in the next seven days for a free car wash. (Bears Express, 5870 Dahlia St., Commerce City; Shell car wash 4650 Peoria St., Denver; Shell car wash 10396 N. Washington St., Thornton; Exxon car wash 2651 E. 120th Ave., Thornton)”

Denver and Adams County fire crews were not deployed to the refinery, dispatchers said, referring queries to Suncor.

The Suncor oil refinery ranks among the state’s largest polluters, emitting more than 800,000 tons a year of heat-trapping greenhouse gases and other pollutants, including sulfur dioxide, hydrogen sulfide, ozone-forming volatile organic compounds, nitrogen oxides and particulates, according to state and federal data.

The Denver Post earlier this year reported that Suncor refinery operators broke a 12.8-ton limit for hydrogen cyanide, an invisible toxic gas that at high concentrations can be deadly. Neither Suncor nor state health officials alerted nearby residents or county emergency managers after a test detected the elevated hydrogen cyanide emissions.