He’s Wanted in a 1970 Bombing. The F.B.I. Aged His Photo to Seek Tips.

In the more than 50 years since a bomb ripped through a mathematics center at the University of Wisconsin, killing a graduate student and injuring several others in what was considered an attack protesting the Vietnam War, the F.B.I. has received hundreds of tips about the whereabouts of one of the suspects, Leo F. Burt.

Just weeks after the attack on Aug. 24, 1970, a waitress in Cleveland was convinced that she had seen him. Years later, another tipster thought he had spotted Mr. Burt at a homeless shelter in Denver. One man said he had seen the fugitive working at a resort in Costa Rica. But Mr. Burt was never found.

Last week, in an attempt to develop new leads, the Federal Bureau of Investigation released new images of what Mr. Burt might look like today, aged to around 75 years old.

Gone are his grin and thick brown hair from an earlier photograph. A frowning Mr. Burt is now depicted graying and balding and, according to the F.B.I.’s remarks beneath the image, may have a mustache, beard or long hair. In one of the images, he wears the same wire-framed round glasses that he wore as a young man. He remains wanted for sabotage, destruction of government property and conspiracy, and “should be considered armed and dangerous,” the F.B.I. said.

Mr. Burt, a University of Wisconsin student in his 20s at the time of the attack, was among four men who the authorities say plotted to bomb the mathematics center in the early hours of Aug. 24, committing the largest act of domestic terrorism at the time.

The four men are said to have detonated a stolen truck containing fertilizer and fuel oil near Sterling Hall, the building that housed the mathematics center, which was affiliated with the U.S. Army. The blast and fire killed Robert Fassnacht, a 33-year-old physics researcher, and injured five others. Investigators at the time said the explosion caused around $6 million in damage.

After the attack, the authorities said the bombers fled to Canada. Three of the suspects, Karleton and Dwight Armstrong, who were brothers, and David Sylvan Fine, were eventually captured.

Karleton Armstrong, a former student at the University of Wisconsin, was arrested in 1972 near Toronto and extradited to Madison, Wis., where he pleaded guilty to arson and second-degree murder and was sentenced to up to 23 years prison.

Mr. Fine, who, like Mr. Burt, was a student at the university, was arrested in 1976. He pleaded guilty and received a seven-year sentence. He was paroled in 1979.

Dwight Armstrong, who was not a student, remained on the run until 1977, when he was arrested and pleaded no contest to a state charge of second-degree murder and guilty to federal charges including conspiracy. He was sentenced to seven years on the state charges and seven on the federal charges, to be served concurrently. He was paroled in 1980 and died in 2010.

The Milwaukee field office of the F.B.I. said it had released the new images to coincide with the 53rd anniversary of the bombing and was offering a reward of up to $150,000 for information leading to Mr. Burt’s arrest. Federal investigators said he was seen leaving the university in a light-colored Chevrolet Corvair.

Mr. Burt was born in Darby, Pa., and, at the time of the bombing, rowed on the university’s varsity team and wrote about sports for the student newspaper. But friends said he had “changed into an avid follower and reporter of the radical movement,” according to a 1970 article in The New York Times.

The police said that two minutes before the bombing, which took place at about 3:45 a.m., they received an anonymous call. “Hey, pig, there’s a bomb in the math research center,” the caller said, according to news reports at the time. The blast tore apart the six-story building, destroying most of its contents, including a $1.5 million computer, and shattering windows up to 10 blocks away from the campus.

In the more than five decades since, the F.B.I. has received hundreds of tips about Mr. Burt.

“But the fugitive has somehow managed to elude capture, leading some to believe he is dead,” the agency said in a news release in 2010. At the time, the agency also issued images of what Mr. Burt might have looked like at that point.