Firefighters get a close look at the damaged remains of a small aircraft, left, Friday inside a building in Austin, Texas. Joseph Stack flew his small airplane Thursday into the building that houses several offices of the Internal Revenue Service.

Austin, Texas ? Robin De Haven was driving the company truck to a job when he saw something that didn’t look right — a small plane, flying extremely low over a heavily congested area of Austin.

The 28-year-old Iraq war veteran recalled Friday how he then saw black smoke billowing from the glass building and rushed to the scene. There, where the plane had exploded into flames in a suicide attack fueled by anti-government hatred, De Haven found five people trapped on the second floor of the burning office housing Internal Revenue Service employees.

“I wanted to go help,” said De Haven, who works for a glass company. “I thought, ‘I’m going to go ahead and do it.’ I thought my boss would understand.”

He quickly hurled his 17-foot ladder onto the building, climbed up and went inside to help the workers escape.

Authorities have credited stories of heroism like De Haven’s for keeping the death toll so low in Thursday’s crash. The pilot, Andrew Joseph Stack III, and one other person were killed when the software engineer fueled with rage against the IRS slammed his plane Thursday morning into the hulking Echelon 1 building.

“When you look at the building, it’s hard to say we were lucky, but we were,” said Ernie Rodriguez of the Austin/Travis County EMS. He credited the actions of “many heroic” people who were at the scene.

Stack, 53, apparently targeted the lower floors of the office building, where nearly 200 IRS employees worked. Thirteen people were injured and one remained hospitalized at an Army burn unit in San Antonio.

Authorities have not identified the other person killed. But in a message to employees Friday, IRS Commissioner Doug Shulman said the agency believes an IRS worker was killed, though he cautioned it hadn’t been officially confirmed.

“It has left many of them and their families anguished and traumatized and seeking answers as to why anyone would commit such a wanton act of violence and why anyone would take such cruel action against innocent men and women,” Shulman said.

In a ranting manifesto posted on a Web site, Stack lashed out at the government — especially its tax code — claiming they robbed him of his savings and derailed his career. In the note, Stack says he realizes “violence not only is the answer, it is the only answer.”

U.S. law enforcement officials were trying to determine if Stack put anything in the plane to worsen the damage. One law enforcement official also said they were looking into whether a marital dispute precipitated the attack. All officials spoke on condition of anonymity because the investigation was ongoing.