A software engineer furious with the US tax agency launched a suicide attack by crashing his small plane into an office building housing nearly 200 employees.
The attack in Austin, Texas, setting off a raging fire that sent the tax workers running for their lives.
At least one person in the building was missing and the pilot was presumed dead, authorities said.
Emergency crews later found a body in the wreckage of the building, but police would not say whether it was the pilot, identified by the FBI as Joseph Stack, 53.
Emergency crews said later they had found a second body in the wreckage.
Austin Fire Department Battalion Chief Palmer Buck said authorities “have now accounted for everybody”, but declined to discuss the identities of those found.
Law enforcement officials said that before taking off yesterday, Stack apparently set fire to his house and posted a long anti-government criticism on the internet. It was dated yesterday and signed “Joe Stack (1956-2010)”.
In it, the author cited run-ins he had with the Internal Revenue Service and ranted about the tax agency, government bailouts and corporate America’s “thugs and plunderers”.
“I have had all I can stand,” he wrote, adding: “I choose not to keep looking over my shoulder at ’big brother’ while he strips my carcass.”
The pilot took off in a single-engine Piper Cherokee from an airport in Georgetown, about 30 miles from Austin, without filing a flight plan. He flew low over the Austin skyline before smashing into the side of the seven-storey, black-glass building just before 10am local time, with a thunderous explosion that instantly stirred memories of the September 11 attacks in 2001.
Flames shot from the building, windows exploded, a huge pillar of black smoke rose over the city, and terrified workers rushed to get out.
The Pentagon scrambled two F-16 fighter jets from Houston to patrol the skies over the burning building before it became clear that it was the act of a lone pilot. President Barack Obama was briefed.
“It felt like a bomb blew off,” said Peggy Walker, an IRS revenue officer who was sitting at her desk. “The ceiling caved in and windows blew in. We got up and ran.”
At least 13 people were injured, with two reported in critical condition. About 190 IRS employees work in the building.
Gerry Cullen was eating breakfast at a restaurant across the street when the plane struck the building and “vanished in a fireball”.
The building, in a heavily congested section of Austin, was still smouldering six hours later, with the worst of the damage on the second and third floors.
The entire outside of the second floor was gone on the side of the building where the plane struck.
Support beams were bent inward. Venetian blinds dangled from blown-out windows, and large sections of the exterior were blackened with soot. It was not immediately clear if any tax records were destroyed.
Andrew Jacobson, an IRS revenue officer who was on the second floor when the plane hit with a “big whoomp” and then a second explosion, said about six people could not use the stairwell because of smoke and debris.
He found a metal bar to break a window so the group could crawl out onto a concrete ledge, where they were rescued by firefighters. His bloody hands were bandaged.
Austin Police Chief Art Acevedo said “heroic actions” by federal employees may explain why the death toll was so low.
The tax protest movement has a long history in the US and was a strong component of anti-government sentiments that surged during the 1990s. That wave culminated in the 1995 bombing of a federal building in Oklahoma City that killed 168 people. Several domestic extremists were later convicted in the plot.