"They did a lot of leg work on this," he said, "it wasn't just taking his tip and going with it."
Chasnick says FBI agents originally sought to dig up a wider swath of farmland, but that the request was whittled down by a judge.
Agents filed a 70-80 page sealed affidavit when seeking the search warrant, Chasnick said.
"It wasn't a quick little thing... I feel bad for them, because it's a case they wanted solved," he said. "I hate that they didn't do the entire area because it leaves a question mark."
FBI Special Agent in Charge Robert Foley publicly acknowledged the dig's failure during a Wednesday press conference.
"With any investigation we consider cost-benefits analysis," Foley said, according to The Associated Press. But, he added, "the FBI and its partners are no corporations. We do not have a profit margin as a bottom line."
Shaykhet, the FBI spokesman, said he didn't know how many times the FBI has tried to dig up Hoffa's remains.
Other failed FBI searches for Hoffa include a 2006 dig at a farm in Milford, Mich., reportedly involving 40 to 50 FBI agents, a 2009 excavation at a Detroit lumberyard and a 2012 search beneath a Roseville, Mich., driveway.