SUPERIOR, Wis. (AP) — A federal agency says a massive refinery fire that forced the temporary evacuation of homes in the coastal Wisconsin city of Superior in April was caused by an explosion that sent debris hurtling into an asphalt storage tank.

The
U.S. Chemical Safety Board
released an
update
Thursday on the
explosion and fire
at the Husky Energy refinery in Superior in late April.

According to the report, the explosion sent a piece of debris about 200 feet (61 meters) into an aboveground storage tank of asphalt. The tank was punctured and spilled hot asphalt, which ignited about two hours later.

The report says 36 people sought medical attention after the explosion, including 11 refinery and contract workers. A large part of Superior was evacuated.

Canada-based
Husky Energy
said in its
second-quarter earnings statement
last week that the refinery is not expected to restart until 2020.