Emergency crews in Texas on Wednesday finally extinguished a massive fire that had engulfed a petrochemicals facility for four days.

Intercontinental Terminals Company (ITC) spokeswoman Alice Richardson said at a news conference that investigators could now begin their probe into what the triggered the massive blaze at the facility in Deer Park, an area just southeast of Houston.

The fire was extinguished Wednesday at 3 a.m. after it had sent a huge, dark plume of smoke thousands of feet into the air when it first began on Sunday.

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The fire briefly flared up late Wednesday afternoon but was contained within 30 seconds by firefighters, the city of Deer Park said in a tweet.

Efforts have now shifted to concerns over air quality and the environmental impact.

The Environmental Protection Agency (EPA) conducted air-quality tests throughout the Houston area, both on the ground and from a small airplane, and "measured no levels of hazardous concentrations," agency official Adam Adams said.

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The EPA also reviewed data collected by ITC, Harris County, where Houston is located, and by the Texas Commission on Environmental Quality (TCEP), which did not show hazardous concentrations of volatile organic compounds, Adams said.

The state environmental agency said in a statement Wednesday that the benzene levels it found near and around the storage facility did not pose health concerns.

Still, some people living in the area have complained of various symptoms since the fire, including headaches, nausea and nose bleeds.

Sema Hernandez, who lives in Pasadena, just west of Deer Park, said all four of her children have experienced headaches since the fire started Sunday. However, she has not been able to take them to a doctor because she doesn't have health insurance.

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"This shouldn't have happened. ... But it did. My question is, what do we do now?" Hernandez said.

The EPA and the TCEQ said they were waiting for test results of water samples to determine any potential impacts from the foam used to fight the fire on waterways next to the storage facility, including the Houston Ship Channel.

The Associated Press contributed to this report.