It took firefighters about an hour to douse a fire at a Costa Mesa office building that sent flames 20 to 30 feet into the air and a plume of black smoke over the 405 Freeway late Tuesday afternoon.

The blaze started in the loading dock of a pharmaceutical company on Sunflower Avenue near where the street ends at the Santa Ana River channel, Costa Mesa Fire Department Battalion Chief Kevin Diamond said.

At least 30 firefighters responded from crews across Orange County, including the Orange County Fire Authority and Fountain Valley, Huntington Beach and Newport Beach fire departments.

Nobody was hurt, and investigators are trying to determine what caused the blaze, Diamond said.

About 70 to 80 people work in the building, which houses mostly administrative operations for Robinson Pharma, said Neil Shah, one of the company's vice presidents.

Many employees had gone home for the day when the fire started shortly after 4:30 p.m., he said, but those remaining were able to get out safely.

"They came very vast," Shah said of the firefighters.

— Jeremiah Dobruck

Twitter: @jeremiahdobruck