NBC DFW submitted a Freedom of Information Act request for the letters.
Investigators say Joseph Andrew Stack, 53, crashed his single-engine plane into the seven-story Eschelon Building on Feb. 18 after setting his house on fire.
A rambling manifesto about problems with the IRS was posted the morning of the crash to the Web site of a software company owned by stack. The letter said the writer spent at least 1,000 hours in 1987 writing, printing and mailing any senator, congressman or governor who might listen to his problems with the IRS.
"None did, and they universally treated me as if I was wasting their time," the manifesto said.
The FBI said the release of the letters or other portions of any files they have could interfere with "enforcement proceedings."
When someone dies, any files the FBI has on the person are generally made public, with some exceptions, such as when the files may be involved in ongoing investigations.
In a telephone interview, an FBI spokesman wouldn't say if the agency had an investigative file open on Stack prior to the plane crash or if a file was opened afterward.
The National Transportation Safety Board released its preliminary report on the crash last week. The agency said there was nothing remarkable in the moments before the pilot's suicide dive into the office building.
The NTSB said the FBI had taken over the investigation.
"As this event was an intentional act, the FBI has assumed jurisdiction and control of the investigation," the report said.
Longtime IRS worker Vernon Hunter , 68, died in the crash. The IRS manager was described as humble and happy with a thirst for life's joys.
Hunter's wife, Valerie, filed a lawsuit five days after the crash to seek unspecified damages. The lawsuit names the estate of Joseph Stack as a potential defendant.
She said the primary goal of the lawsuit is to get an injunction against the release of the full autopsy report for Hunter. Attorney Daniel Ross said Valerie is seeking to protect her children and grandchildren by keeping those details private.
There is no word from the FBI on when its investigation will be complete.
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- Arson Ruled in Joe Stack House Fire
- IRS Worker to be Buried Friday
- Stack's Daughter Says He's a Hero
- Hundreds Gather to See IRS Attack Damage
- Stack's Wife Issues Statement; Victim Identified
- "He Was a Good Man, He Really Was"
- Friends Unaware of Stack's Anti-Government Rage
- Manifesto Attributed to Austin's Joseph Stack
- Crews Recover 2 Bodies in IRS Wreckage