Austin, Texas (CNN) -- The man who flew an airplane into a building housing an Internal Revenue Service office may have replaced some of its seats with a drum of fuel to cause maximum damage, a law enforcement official familiar with the investigation said Friday.
The official, who would not speak on the record because it is an ongoing case, said investigators have determined that the Piper Cherokee PA-28 had several seats removed and that a fuel drum was missing from the airport from which Andrew Joseph "Joe" Stack III took off Thursday morning.
"I think there is a good chance he might have put it on his plane," said the official, who cautioned that investigators were still working that lead and sifting through the crash site.
The single-engine plane has a fuel tank capacity of 38 gallons and is equipped with four seats, according to the Web site risingup.com.
The FBI said Friday it has taken the lead role in the investigation of Thursday's crash into the 7-story building in northwest Austin that held offices for nearly 200 IRS workers.
"You're talking about a federal agency that was basically assaulted," Ralph Diaz, special agent in charge of the FBI's San Antonio, Texas, field office, told reporters.
Two people were killed and two others were hospitalized, federal officials said.
Though the remains found in the IRS building have been identified, their identities will not be revealed until after a forensic examination, Diaz said Friday.
"One may be Mr. Stack and our fear is certainly the other would be either someone who was visiting or someone who was employed in the building," he said.
Emergency services chief Ernie Rodriguez said one of the injured was treated and released; the other remained hospitalized.
City of Austin Fire Chief Rhoda Mae Kerr credited the building's fire protection, fast work by the Fire Department and the fact that the employees had practiced exiting the building in case of an emergency for the low loss of life and injuries. "It truly worked," she said.
Rodriguez said units arrived on the scene within five minutes of the first 911 call prepared to manage hundreds of injured people. "We found only two persons," he said.
"When you look at the fire, when you look at the evidence, it's hard to say that we were lucky, but we were," he said. "The big thing to remember is that yesterday, in the midst of this event, God's grace was upon us."
Austin Police Chief Art Acevedo credited fast thinking by some people who were inside the building as the plane approached for the fact that few people were hurt. "Some folks saw it coming and sounded the alarm yesterday, and some folks started running away from that side of the building," he said.
Stack's wife, Sheryl Stack, expressed her "sincere sympathy to the victims and their families" Friday.
"Words cannot adequately express the sorrow or the sympathy I feel for everyone affected by this unimaginable tragedy," she said in a statement read by a family friend, Rayford Walker.
Authorities say Stack also torched his $230,000 home in Austin on Thursday morning before embarking on his fatal flight.
Police said Sheryl Stack spent Wednesday night in an Austin-area hotel but did not say why. Acevedo said police had received no calls of domestic violence from the house. The only calls to police were made a couple of years ago and concerned barking dogs, he said.
A 3,000-word message on a Web site registered to Stack railed against the government, particularly the IRS.
Read the apparent suicide note (PDF)
"I saw it written once that the definition of insanity is repeating the same process over and over and expecting the outcome to suddenly be different," the online message says. "I am finally ready to stop this insanity. Well, Mr. Big Brother IRS man, let's try something different; take my pound of flesh and sleep well."
Acevedo reiterated on Friday his assertion first made Thursday that the incident was not an act of terrorism. "Part of our jobs in law enforcement is not to overreact and cause undue panic," he said. "And with the information that we had, there was no need to alarm our colleagues around the country and community members by using the word 'terrorism.' That is why definitely I did not use it yesterday and I'm not using it today."
An IRS criminal investigator visited Stack's accountant after Thursday's events, leaving a business card on the CPA's front door. The online message believed to have been written by Stack mentions accountant Bill Ross, saying it had become "brutally evident that he was representing himself and not me."
Watch what Stack's writing reveals about him
IRS investigator James L. Neff said that Ross is not under investigation; officials just wanted to make sure he was OK. "We didn't know if he was alive or dead," Neff said.
Ross, who did not return a call from CNN, was fine, the investigator said.
Friends and former colleagues said Friday they had no inkling of the rage apparently building inside Stack.
"He hid that very well," said Billy Eli, in whose band Stack played bass until a few years ago. "Obviously he was in some serious distress and had some real despair. I never saw that."
Neither did another former bandmate, Ric Furley.
Watch how bandmates remember Stack
"I never saw him in a bad mood or speaking negatively about anything or anyone," Furley told CNN's "American Morning."
"This has been such a shock because it was totally out of character with the Joe Stack I played with for three years," said Eli.
"We liked him," said Furley. "We liked him."
Share information about the crash and the pilot
An IRS spokesman said federal law prohibits the agency from releasing information about any interactions the agency may have had with Stack.
The IRS issued a statement Friday saying tax returns will not be delayed.
CNN's David Mattingly, Ed Lavandera and Carol Cratty contributed to this report.