Webb, who would have been 86 on Friday, is accused of beating and fatally shooting Saxonburg police Chief Gregory Adams during a December 4, 1980, fight in the Butler County borough.
Thomas King, a lawyer for Adams' widow, said in an email that he was told by prosecutors that Webb's former wife had told authorities about the remains in exchange for immunity.
She led investigators to her home in Dartmouth, Massachusettes, where human remains believed to be Webb's were found, the AP report said.
An autopsy was being performed Friday to establish if they are the remains of career criminal Donald Eugene Webb.
Adams's widow, Mary Ann Jones, told the Associated Press last month that Federal Bureau of Investigation agents informed her Donald Webb may have hidden out in the Dartmouth home during brief stints in the 1990s.
The FBI announced Friday that remains of one of agency's most-wanted fugitives have been located after a 37-year search.
Adams' murder is the longest-running cold-case involving a police chief in the United States, CBS noted. "Although it's unfortunate Mr. Webb will never be brought to justice to pay for his crimes, we're hopeful the family can find some closure in knowing that this alleged murderer has been located", said Harold H. Shaw, special agent in charge of the FBI Boston division.
Investigators say Webb shot Adams during a traffic stop.
Mary Ann Jones, Adams' widow, told Channel 11 she didn't realize how emotional it would be to relive all of this.
"She never gave a thought to me and my family".
Police have discovered what appeared to be human remains behind a home with connections to the decades-old, unsolved slaying of a Pennsylvania police chief, authorities said. He was added to the list on May 4, 1981, and removed from it on March 31, 2007.
Authorities began searching the backyard of Maplecrest Drive in North Dartmouth, Massachusetts on Thursday around 11 a.m. State police in MA and Pennsylvania also were on hand for the dig.