Denver firefighters rescued a construction worker who was trapped Thursday afternoon when a wall collapsed on him at a school building that was being renovated.

About 2:50 p.m., the man was loaded into an ambulance. He gave a “thumbs up” sign to onlookers.

“He’s conscious, he’s thankful to the firefighters who did such a wonderful job with him,” said David White, vice president of Swinerton Builders, the general contractor .

It was not immediately known what the wall was made of, but most of the structure at 150 S. Pearl Street is brick, White said.

The man was working on the second floor of the building when the wall collapsed, White said. The building is four stories tall.

An investigation is ongoing to determine what caused the collapse.

The incident happened about 2 p.m., said Mark Watson, fire department spokesman.

Formerly Byers Middle School, the building is being renovated for the Denver School of Science and Technology, a charter school.

The work to free him was slow and methodical because rescuers wanted to make sure that other parts of the room don’t collapse as well.

There are no other reports of injuries.

The Byers building was originally constructed in 1921, according to a Denver Public Schools web page.

It served as a junior high and middle school for many years before becoming the Denver School of Arts. DSA left the building in 2003 and it has been vacant since