Dr. Trent P. Pierce, 54, remained in critical condition after an explosive device planted in his Lexus blew up as he left for work.
Pierce lost an eye and suffered severe burns and shrapnel injuries in Wednesday's attack in his driveway in West Memphis, Ark.
A family friend, Scott Ferguson, said Pierce's family told him he'd bent down to move a tire blocking his hybrid Lexus SUV minutes before the explosion.
Austin Banks, a senior special agent with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators want to learn more about the tire.
"This is a lead we're going to follow up on," he said.
Banks said residue and remains from the bomb had been sent to the agency's laboratory in Atlanta. Banks described the bomb as homemade, not manufactured dynamite or a military-style explosive.
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