Mexico: A massive explosion at an oil facility in southeastern Mexico killed at least three people and injured 136, sparking panic as it generated a huge plume of toxic smoke, officials said.

The mid-afternoon blast Wednesday at the petrochemical plant in the city of Coatzacoalcos was felt from as far as 10 kilometers (six miles) away and forced the evacuation of nearby schools and businesses.

The cause of the incident at the Petroquimica Mexicana de Vinilo (PMV) plant was still under investigation with initial clues suggesting there was a “leak of flammable compounds,” said Jose Antonio Gonzalez, chief executive of state-run energy giant Pemex that jointly owns the facility.

Veracruz state governor Javier Duarte told Milenio Television that the blast killed three workers at the plant.

But Gonzalez warned there could be more casualties.

“It is very possible that the numbers will go up,” he told Televisa.

The number of injured rose steadily to 136 as the extent of the blast became known, with 88 still hospitalized late Wednesday, said Gonzalez, who was sent to Coatzacoalcos by President Enrique Pena Nieto.

Thirteen were in serious condition. But 48 people with minor injuries had already been sent home.

A “cloud of toxic chemicals” spewed from the plant following the explosion, with footage from the scene showing thick, black smoke billowing from the facility.

Some 100 plant workers and about 2,000 residents had to be evacuated, according to Duarte.

Duarte rushed to the scene of what he said was “a very strong explosion,” where fire crews had the blaze under control.

As the situation unfolded, he urged people living in the vicinity to remain indoors as the “cloud of chemicals” dissipated.

School classes in Coatzacoalcos, a port city, and nearby communities were suspended.

Pemex said in the late afternoon that the situation was “completely under control.”

– Windows shattered –

Several stores in Coatzacoalcos had their windows blown out by the force of the blast.

As night fell, residents tried their best to get over the initial shock.

“I left my house running, I thought the whole city was going to catch fire,” said Marcela Andrade Moreno, a resident whose windows were shattered by the blast.

Other terrified residents also rushed out of their homes, fearing the repeat of a 1991 explosion at the same facility that sparked a gas leak. The death toll from that incident officially stands at six, although local media insist the number is much higher.

“We live in a time bomb,” said Abelardo Garduza, a merchant from the village of Allende located a few of kilometers from the plant.

Pena Nieto said on Twitter that the government would help the “affected workers and neighbors of the area.”

Fires at oil facilities in Mexico are fairly regular occurrences.

In February, two people were killed and at least seven injured in a blaze at a Pemex oil platform off the coast of Campeche, also in the southeast.

In August 2015, a gas pipeline belonging to Pemex exploded in the city of Monterrey killing five people.

Pemex provides one-fifth of the Mexican government’s revenue but has posted huge losses amid crumbling production and oil prices.