HOUSTON (AP) - Potentially hazardous chemicals spilled Friday at a Houston-area petrochemical tank farm where a dayslong fire damaged several tanks, the company that operates the farm said.

The breach occurred before 12:30 p.m. in a containment dike near a damaged tank that crews were trying to drain of flammable product that includes benzene, said Dale Samuelsen, a spokesman for Intercontinental Terminals Company. An estimated 20,000 barrels of liquid were in the tank, which was one of several to catch on fire.

Samuelsen said the company asked its neighbors, including other industrial sites and the nearby San Jacinto Texas State Historic Site, to shelter in place. He said the company was trying to find out what chemicals were in the dike that breached.

The tanks on site contain components of gasoline and materials used in nail polish remover, glues and paint thinner.

Authorities had not asked residents in surrounding Deer Park to shelter in place early Friday afternoon. People living near the plant in Deer Park were told Thursday to remain indoors after air monitors detected elevated levels of benzene. The order was lifted later Thursday.

Adam Adams of the Environmental Protection Agency said authorities believe the other tanks that burned on site were stable.

The fire began Sunday and was extinguished Wednesday. It caused a giant plume of black smoke that billowed over much of Houston.

Area schools canceled classes as tests on Thursday indicated higher-than-normal levels of benzene in the air. Benzene is a highly flammable liquid found naturally in gasoline and crude oil. It evaporates quickly and can cause drowsiness, dizziness, and headaches.

Adams said air tests by the U.S. Environmental Protection Agency and the company had not shown any positive results for high levels of benzene Friday. One positive test after 4 a.m. from a sensor operated by Harris County was verified to be a false alarm, a county spokeswoman said.

Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.