Fire investigators were still sifting through ashes and soot Wednesday to determine what caused a fire that flattened a 21/2-acre warehouse used in a Christmas tree business.

The warehouse at 7150 S. State St., owned by Allied Development Co., started burning about 2:35 p.m. Tuesday. The three-alarm blaze spread quickly through the plywood-covered, wood-frame buildings, spewing a black column of smoke above the Salt Lake Valley, said Midvale Fire Chief Merrill Ross.One Midvale firefighter was treated at Cottonwood Hospital for minor burns, then released. No other injuries occurred, Ross said.

Fearing a 500-gallon tank filled with gasoline and a 250-gallon propane tank might explode, officials blocked off a 10-block section of State Street and evacuated businesses and homes in a 1,500-foot radius around the fire. The two tanks were isolated and cooled to prevent an explosion, Ross said.

High temperatures and gusting winds hampered firefighters' efforts to control the blaze. Of particular concern were hot brands blowing north onto homes on adjacent Glover Avenue (7065 South).

"With a fire of that magnitude there is always a concern to keep it controlled," Ross said. "The firefighters have taken a hard beating. This heat really causes some problems."

The closure of State Street and nearby freeway exits caused traffic jams on major arteries throughout the south end of the valley. On I-15, all lanes of traffic were at a virtual standstill from 53rd South to 72nd South for about two hours, a Utah Highway Patrol dispatcher said.

The evacuation inconvenienced some people, including bank employees, patrons at a hair salon and Glover Avenue residents. Residents were allowed to return about 5 p.m.

Robert and Lisa Vanfleet said they were ordered away from their home at 17 W. Glover Ave. and were given just enough time to close all of the windows to keep the billowing smoke from filling the house.

Animals from a pet store south of the warehouse were probably happy to get a change of scenery.

"We got everything out except the fish," Ross said.

While there was no official report, Frank Lathrop, manager of Allied Christmas tree operations, said he estimates the damage at $250,000.

Thousands of Christmas trees are flocked in the large warehouse each November and December for shipment to 28 lots along the Wasatch Front. The warehouse goes unused during the rest of the year. Some empty ammunition boxes, plastic tree stands and bags of tree flocking were damaged or destroyed in the fire, Lathrop said.

"There was nothing of great value," he said.

Neighbors said they heard two loud booms during the fire. Ross speculated the sounds were made by explosions of portable kerosene heaters also stored in the warehouse.

The fire caused some anxious moments for Midvale City Engineer Duane Goodyear, who said the firefighters quickly drained water from city storage tanks. Connections were opened with Sandy's water system and the Salt Lake County Water Conservancy District lines, and the flow of a reserve pump was increased to meet the demand, which reached 7,000 gallons a minute during the blaze.

Merchandise at Surplus Saver, a military surplus store, which rents a streetside steel building and warehouse on the Allied property, received only smoke damage from the fire.

Some 75 firefighters responded to the blaze from five south valley fire departments. The fire was under control at 4:10 p.m. Investigators from the state fire marshal's office and Salt Lake County Arson Task Force are investigating the cause of the blaze, Ross said.