"What's left is what we call just a shell," said Kimball. "We're going to have a concrete slab and start from scratch."
Repairs on the 64,000-square-foot building should be completed within six months, he said.
Andrew Joseph Stack III on Feb. 18 flew his single-engine plane into the multistory structure. IRS worker Vernon Hunter also was killed in the fiery crash.
Stack, involved in a tax dispute for several years, left behind documents expressing his anger at the IRS, which occupied the top three floors of the building.
The IRS, which had about 200 employees at Echelon I, has signed a two-year lease on a 47,000-square-foot office space elsewhere in Austin, said Lea Crusberg, an IRS spokeswoman. She declined to say where it was located.
"We weighed a number of factors into the decision to lease the new facility. We believe this decision is in the best interest of our employees and taxpayers," Crusberg said via e-mail. Local taxpayers will be contacted by the IRS to set up an appointment and be advised of the new address.
The Echelon I had four stories and a reflective glass exterior.
"We're getting ready to take off the whole glass curtain wall, because it's from the 1980s, and it would be too much to try and match and save it," Kimball told the Austin American-Statesman. "So the owner's looking at changing it. It'll probably end up looking different than the other three (Echelon) buildings (in the complex), with a different type of glass."