Oakland Township, Michigan (CNN) -- The search in a field near Detroit for ex-Teamsters boss Jimmy Hoffa ended Wednesday, and as with so many past attempts to find him, authorities walked away empty-handed, the FBI said.
Authorities resumed digging in the field Wednesday morning, and a law enforcement source said the search was expanded to a site where a house once stood.
John Anthony, a former FBI special agent involved in the original 1975 hunt for Hoffa, said the tip from alleged mobster Tony Zerilli which prompted this week's renewed search was the most credible he's heard since Hoffa's disappearance -- though he still has his reservations, he said.
Anthony said he's skeptical of Zerilli's claim that Hoffa was killed with a shovel because it wasn't the way of organized crime members in those days, who Anthony said would have preferred to shoot him in the head at close range.
Asked if it was possible Zerilli was coming forward for monetary reasons, Anthony said, "I think there's no doubt about that. The man's broke. He has no money. That's why he came out in January with this story on the insistence of his wife."
A second day of digging in the field Tuesday yielded no sign of the remains of the former Teamsters boss.
Agents began digging Monday in waist-high grass in Oakland Township, north of Detroit, a location determined in part from information provided by Zerilli. Media and curious onlookers gathered some distance from the private property.
Oakland Sheriff Mike Bouchard said investigators were using probes to determine what the ground makeup is, but had not found samples that would require lab analysis.
Two concrete slabs were removed during the dig. It's unclear whether the slabs were foundations for a barn that once stood there.
Scientists from Michigan State University were at the site Tuesday to help with soil analysis.
This is the latest chapter of the nearly four-decade search for Hoffa. It was sparked by "highly credible" information from Zerilli, according to a law enforcement source with direct knowledge of the investigation.
Earlier this year, Zerilli, now in his 80s, told New York's NBC 4 that Hoffa was buried in a Michigan field about 20 miles north of where he was last seen in 1975.
Hoffa, then 62, disappeared after being seen on July 30, 1975, outside a Detroit-area restaurant. The FBI said at the time that the disappearance could have been linked to Hoffa's efforts to regain power in the Teamsters and to the mob's influence over the union's pension funds.
Zerilli's attorney, David Chasnick, told reporters Monday that Zerilli was told Hoffa was hit with a shovel and buried alive.
Zerilli published a manuscript about the Hoffa claim online that includes details of the alleged hit.
"He wasn't shot, he wasn't stabbed, nothing like that. A cement slab of some sort was placed on top of the dirt to make certain he was not going to be discovered. And that was it. End of story," Zerilli's manuscript says.
The FBI spent months looking into Zerilli's claims before seeking court authorization to excavate the field and look for evidence of a shallow grave, according to a law enforcement source.
Contrary to what's been thought for years, Zerilli said he was told Hoffa's disappearance was not connected to Anthony "Tony Pro" Provensano, the New York City-area Genovese family crime boss who allegedly wanted to get rid of Hoffa.
Instead, according to the source, Zerilli -- convicted years ago of crimes in connection with organized crime in Detroit -- told the FBI that Detroit mobsters wanted Hoffa dead.
At the time, Hoffa was thought to be trying to get back into a power position with the labor movement after his release from prison. He was convicted in 1967 for jury tampering and fraud. President Richard Nixon pardoned him in 1971.
Zerilli was in prison himself when Hoffa disappeared.
Zerilli, according to the law enforcement source, said that when he was freed, he asked a mob enforcer what happened to Hoffa.
The mobster allegedly told Zerilli that Detroit's crime bosses ordered the Hoffa hit. They lured him to a meeting and then drove him to a farm owned by a mob underboss. The enforcer allegedly told Zerilli that Hoffa was killed and buried on the property, which covers several acres.
The area being searched was described as relatively small, about the size of a small party tent, according to the source. Aerial video showed a somewhat larger area had been cleared of grass.
Zerilli has been to the site more than once, said Chasnick, who declined to elaborate.
Hoffa's daughter, Barbara Crancer, told CNN by phone Tuesday that she is always appreciative when the FBI follows credible leads in the case.
But, she said, she doesn't want to get her hopes up.
"We've been through it too many times," she said.
FBI Special Agent Bob Foley, head of the agency's Detroit office, told CNN at the scene that the information leading to the search "reached the threshold of probable cause, which was sufficient to allow us to obtain a search warrant." The paperwork supporting the search warrant is under seal.
"If it didn't rise to that level then, certainly, we wouldn't be out here," Foley said.
Hoffa's disappearance and presumed death have vexed investigators. As recently as October, soil samples were taken from a home in a suburban Detroit community after a tipster claimed he saw a body buried in the yard a day after Hoffa disappeared.
The soil samples were tested, and showed no evidence of human remains or decomposition.
Zerilli was freed in 2008 after his last prison sentence. Keith Corbett, a former U.S. attorney, told CNN earlier this year that Zerilli headed a Detroit organized crime family from 1970 to 1975, but was in prison when Hoffa vanished.
CNN's Yon Pomrenze, Laura Batchelor, Sheila Steffen, Poppy Harlow and Jason Hanna contributed to this report.