An equipment failure at Suncor Energy’s oil refinery north of Denver led to a burst of opaque yellow and hydrogen sulfide pollution Tuesday, triggering a company “vapor release alarm” and prompting operators to shut down a unit and monitor emissions in surrounding neighborhoods.

This happened less than two weeks after Colorado health officials announced a negotiated legal settlement with Suncor for previous violations — an agreement that officials hailed as an opportunity to “reset” a pattern of pollution problems at the refinery.

On Wednesday, state air pollution control officials said they’re investigating this latest incident for possible future penalties and called for a complete report from Suncor within 30 days. And state regulators are counting on remedies laid out in the settlement, such as a requirement that Suncor hire a third-party consultant to determine root causes of repeated equipment failures at the refinery, said Andrew Bare, spokesman for the Colorado Department of Public Health and Environment’s Air Pollution Control Division.

Suncor sent air pollution officials a notice saying initial company investigations detected “excess opacity from one of the fluidized catalytic cracking units and excess hydrogen sulfide from one of the plant flares,” Bare said.

Hydrogen sulfide can be deadly. But state officials have reviewed some of their own data from state-run monitoring stations and found that pollutant concentrations were well below National Ambient Air Quality Standard limits.

Highly visible yellow clouds billowed out from the refinery starting around 5:40 p.m. Tuesday, spreading mostly “clay-like” catalyst material. Suncor crews blocked off Brighton Boulevard temporarily as they shut down malfunctioning equipment.

A Suncor statement posted on social media after the incident said: “All employees are accounted for and community air monitoring is in place.”

Thick, yellow smoke is billowing from the refinery complex in Commerce City. Curiosity slowing on 270. #cotraffic @KOAColorado pic.twitter.com/oCFbFFBiNJ — John Morrissey (@KOAJetcopter) March 17, 2020

Adams County Commissioner Steve O’Dorisio, who lives slightly more than 2 miles from the refinery and drove to the scene, raised concerns that Suncor did not provide crucial facts in an initial notice to county emergency managers.

“I’m concerned that the information doesn’t say the severity or how people should respond. We don’t know if people were placed at risk or need to do anything as a result of this incident. We need more information. We need information people can use to determine if they should take any action,” O’Dorisio said.

“As a member of the community and as a leader, I am concerned that I still don’t know how to respond when I get this type of information that really doesn’t tell us anything.”

The smoky emissions this week marked the latest in a series of incidents at the refinery, in Commerce City, just north of Denver. Equipment failures led to similar operational upsets Dec. 11 and Nov. 27, according to CDPHE and company statements. Refinery workers were moved to safe zones.

State air pollution control officials said they were aware of the equipment failure within an hour of the incident and were monitoring the refinery Tuesday night. Typically, CDPHE air officials respond by reviewing air-monitoring data collected at state-run pollution monitoring stations placed around the refinery.

Suncor shut down the failing unit and notified state officials at 6:05 p.m. that one of its air blowers failed. “State officials do not believe there is an ongoing risk to the community,” Bare said in a statement Tuesday night. “Air blowers keep air and catalyst circulating. This failure resulted in highly visible opacity emissions.”

Inside the plant, Suncor crews used handheld devices to measure various materials, including hydrogen sulfide and sulfur dioxide.

“We know that the air blower in one of the refinery’s fluidized catalytic cracking units failed. We do not know yet why it failed,” Bare said Wednesday night in a statement that referred to Colorado’s legal settlement provisions requiring a third-party investigation of problems.

“We expect that the refinery will be a more compliant facility once these and other provisions of the settlement are put in place. We will also be investigating this incident thoroughly to determine lawful and appropriate steps to provide accountability and address root causes.”

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 state sets limits for how much Suncor can emit that are listed in an air pollution permit. Canada-based Suncor has had problems for years despite investing heavily to keep the refinery running.

This month, state public health officials announced the negotiated settlement with Suncor that also requires the company to pay administrative penalties and embark on projects — to resolve state complaints about multiple air-pollution incidents between 2017 and 2019 at the refinery. Suncor must pay $1 million in penalties to the state and an additional $426,705 for other penalties to the state and the U.S. Environmental Protection Agency, state officials said in announcing what they called an “historic” settlement for a single facility. An additional $2.6 million must be paid to fund community environmental projects, state officials said.

The Denver Post last 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.

The Colorado Latino Forum, representing north Denver and other residents near the refinery, issued a statement of concern lamenting “yet another incident at Suncor” and “a tepid response by Suncor.”

Community members “are left with questions,” said the statement by forum co-chair Ean Tafoya.

“Until samples are collected and analyzed by CDPHE, we won’t be certain that no harmful chemicals were released … With the COVID-19 response, are we certain that CDPHE staff are available and prepared for a response? Is anyone hurt? … How are they notifying the surrounding community? A siren and an executed plan? We have been pushing to build a plan by and for the people with Suncor and all the necessary government agencies. The community deserves a better plan and CLF is committed to ensuring its creation.”

A Suncor social media update on Wednesday noted that “this is the same unit that experienced an issue on December 11, 2019, during start-up.” Suncor officials wrote that, after the December incident, “we committed to do better. Clearly, yesterday is not an example of doing better, and we have more work to do.”

Suncor tested air inside and downwind of the refinery. “The data has not shown any readings of concern,” the statement said.

“This was a serious event and we are giving it our full attention. We have launched an investigation to determine the exact cause of the air blower trip. We need to understand this information prior to a safe re-start.”