A software engineer furious with the US tax agency has ploughed his small plane into an office building housing nearly 200 employees, setting off a raging fire that sent workers fleeing as thick plumes of black smoke poured into the air.

The pilot was presumed dead following Thursday’s crash and emergency crews have now found a second body in the wreckage where the plane hit the building.

Fire Department Battalion Chief Palmer Buck said late on Thursday that authorities "have now accounted for everybody" but declined to discuss the identities of those found.

A US law official identified the pilot as Joseph Stack - whose home was set on fire just before the crash - and said investigators were looking at an anti-government message on his website.

The website outlines problems with the Internal Revenue Service and says violence "is the only answer".