A construction worker's blow torch caused the three-alarm blaze that snarled traffic last week on the Throgs Neck Bridge, a spokesman for the city Fire Department said Monday.

The blaze, which is still being investigated, was accidental, spokesman Steve Ritea said.

The fire broke out about 5 a.m. Friday in scaffolding and construction materials beneath the bridge on the Queens side.

Work being performed under the bridge at the time of the blaze is part of a reconstruction project to replace about 140,000 square feet of roadway deck at the Queens approach.

E.E. Cruz & Co. was awarded the $96.7-million roadway deck contract in 2008. The Holmdel, N.J., company subcontracted to Imperial Iron Works of the Bronx for the work scheduled the day of the fire, MTA Bridges and Tunnels spokeswoman Joyce Mulvaney said.

E.E. Cruz has been involved with other large projects in the city, including the reconstruction of Ground Zero, according to the company's Web site.

Supervisors for both companies didn't return phone messages Monday seeking comment about construction on the Throgs Neck.

The bridge remained closed to Bronx-bound truck traffic yesterday, while all Queens-bound lanes were free from restriction, the Metropolitan Transportation Authority said.

Most trucks are being diverted to the RFK-Triborough Bridge and the Bronx-Whitestone Bridge until further notice, Mulvaney said.

Mulvaney didn't know how long the restrictions would remain on the Throgs Neck.

Construction crews and engineers continued Monday to assess the fire damage.

The fire occurred at the south end of the bridge, near where the Cross Island Parkway entrance merges with the Clearview Expressway.

Crews were apparently welding a section of the roadway deck when the fire began. Once completed, the project will provide a new steel-reinforced concrete deck, the MTA said.

The Queens-Bronx span, which opened in 1961, carries about 112,000 vehicles daily.