A multi-million-dollar fire at a UC Santa Cruz laboratory has been deemed an accident, though investigators say they may never know exactly what caused the blaze.

Three weeks after the Jan. 11 early morning inferno destroyed the fourth-floor laboratory and most of the RNA genetic research of professor Manuel Ares at Sinsheimer Labs on Science Hill, cleanup crews were removing the scorched plaster, cement and metal from Ares’ lab and a vacant lab that was destroyed next door.

Investigators believe the fire was unintentional and started under a bench near the south end of Ares’ lab, UCSC fire chief Charles Hernandez said. A preliminary report lists wastebaskets found under benches that could have contained paper towels with flammable liquids on them or the use of lab equipment the day before as possible causes of the blaze.

The lab and the hallway outside are blackened with soot and smoke damage. Hernandez pointed out the scorch marks on the 15-foot-high ceiling where the 1,600-to-1,800-degree flames reached. The smell of smoke hung thick in the air in the fourth-floor and the third-floor labs below.

All of the building was damaged in some way by fire, smoke or the 5,000 gallons of water used to extinguish the flames, said Dave Kliger, dean of the UCSC natural sciences program. Large tubes are pumping dry air into the building to prevent mold growth from all the water damage, Kliger said.

Sensitive equipment in labs, such as Kliger’s first-floor optical lab, is being removed and cleaned. A seminar room has been converted into a cleaning station, Kliger said, and after each piece of electronic equipment is cleaned it is tested to see the extent of damage. The building contains millions of dollars worth of equipment, Kliger said. He estimated that 10 percent of that equipment is damaged beyond repair.

University spokeswoman Liz Irwin said the total damage estimate for the building is $4 million to $5 million. Campus engineer Frank Zwart said it could take five weeks to reopen some of the lower labs damaged by smoke and water. The fourth-floor labs destroyed by the blaze will have to be completely rebuilt, Zwart said, and he has no time estimate on how long that will take.

Contact Jason Schultz at jschultz@santa-cruz.com.