HOUSTON (Reuters) – Firefighters late Tuesday contained a raging inferno at a Mitsui & Co petrochemical storage site outside of Houston that has been billowing acrid smoke for days as Texas officials and environmental groups raced to expand air monitoring.

The blaze at Mitsui unit Intercontinental Terminals Co in Deer Park, Texas, was reduced to four burning tanks from eight, a Harris County official said in a statement, after additional equipment and firefighters trained in handling tank fires arrived earlier in the day.

The fire began on Sunday when a leaking tank containing volatile naphtha, a fuel used in the production of gasoline, ignited and flames quickly spread to other tanks, ITC said. By Tuesday morning, the fire had ignited 12 of 15 tanks.

The tanks each hold up to 80,000 barrels, or 3.3 million gallons, of volatile liquid fuels, making the fire difficult to extinguish. There were no employees or firefighters injured since the blaze began, an ITC spokesman said on Tuesday.

Thick, acrid smoke could be smelled miles away in Houston and was visible dozens of miles away. State and federal monitors said air quality was safe, but environmental groups disagreed and said they would conduct their own monitoring.

The Texas Commission on Environmental Quality reported an increase in soot and other contaminants at ground levels around the site but said levels remained below those considered unhealthy. Monitoring by an Environmental Protection Agency aircraft also found “no significant detections,” the EPA said.

“All the monitors are indicating no risk right now and they are looking at particulates,” said Ryan Sitton, a commissioner with the state’s energy regulator, adding there were no toxins released by the fire. Soot particles “are not at a level that causes risk to people,” he said.

But Neil Carmen, a director at the Texas chapter of the Sierra Club environmental group, said the airborne plume likely contained tens of thousands of milligrams of particles, well above levels considered safe.

The Environmental Defense Fund (EDF), a nongovernmental organization, was deploying 10 air-quality monitors to check for nitrogen oxides and soot around Houston and adding another 10 monitors in the near future, said Matt Tresaugue, an EDF spokesman. He said the city had requested the EDF monitors.

Just as the company had struggled to contain the blaze, it has changed its explanation for the fire’s spread and the number of tanks involved. On Monday, it said 8 tanks were burning then revised the figure to seven. Tuesday morning it blamed a failure of two water pumps for the fire’s expansion to more tanks, and by the afternoon said there were no pump failures.

Gasoline prices on the Colonial pipeline, which sends fuel to the U.S. East Coast from Houston, were up on Tuesday between 1 and 2 cents a gallon over levels prior to the fire.

A local fire official said the blaze may have to burn itself out. “I can’t tell you how long it will take to burn out,” said Harris County Fire Marshal Laurie Christensen at a morning briefing. “I’m not going to give you a timetable.”

Some of the water and chemicals have washed into the adjacent Houston Ship Channel that links the Gulf of Mexico to Houston, the nation’s busiest petrochemical port, ITC spokesman Dale Samuelsen said.

On Tuesday, ITC added a 15-person crew from US Fire Pump that has experience battling tank-farm fires as well as additional high-pressure pumps and suppressant foam, spokesmen said.

“We have been up to this point in defensive mode” trying to contain the fire, said Samuelsen. “Because of the expertise these guys bring, the expectation is we’ll be able to go into offensive mode.”

Samuelsen said the burning tanks are within a six-foot tall earthen berm that is collecting water and chemicals. Firefighters are pumping 10,000 to 20,000 gallons of water and foam a minute onto the tanks.