A massive explosion that leveled a major Houston underground tunnel and sent thousands of people scrambling for cover on Friday left at least two people dead and dozens more injured, officials said.

The blast occurred about 9:15 a.m.

(815 GMT) at the intersection of I-45 and I-10 in the north Houston suburb of Katy, the Houston Fire Department said in a statement.

Emergency crews were using excavators to clear a large pile of debris that had been piled up on the freeway that cuts through the city.

The Houston Chronicle reported that one person was killed in the blast.

It was not immediately clear how the explosion happened.

A statement from the Houston County Sheriff’s Office said investigators were working to determine what caused the explosion.

The explosion caused a significant amount of damage, according to the Houston Police Department, but officials were still determining the extent of the damage.