In a last-ditch attempt to reduce Tom ­Petters’ 50-year sentence for running a $3.65 billion Ponzi scheme, an attorney for the former Wayzata businessman asserted Monday that Petters was unaware of a purported 30-year sentence offer until after his conviction nearly four years ago.

Attorney Steve Meshbesher, who did not represent Petters in the criminal case, argued that his client deserves a hearing to determine whether he is entitled to the lesser sentence.

“Defendant Petters affirms that the very first time the 30-year cap was ever raised was during a meeting in the U.S. marshal’s holding cell immediately following his trial and conviction,” Meshbesher wrote in a brief filed with trial Judge Richard Kyle.

Petters quotes defense attorney Jon Hopeman as saying while he was in the holding cell, “Well, we had to go to trial [because] we could only get you a 30-year minimum deal.”

Meshbesher’s brief is the last of a series of briefs to be filed on the sentencing dispute and the matter is now in Kyle’s hands. Kyle can make a ruling based on the existing briefs, request further briefs or conduct a court hearing that could include testimony before making a ruling.

Both the Petters defense team and federal prosecutors have strenuously argued previously that Petters was well aware of the potential 30-year cap on a prison sentence and rejected it multiple times.

Chris Madel, an attorney representing Hopeman, defense attorneys Paul Engh, Eric Riensche and investigator William O’Keefe, said Monday that Petters’ most recent assertions contradict the sworn statements of each member of the defense team.

“All four said they individually had personal conversations with the defendant” about the 30-year cap, Madel said.

Petters, 55, is serving a 50-year prison sentence at the federal penitentiary in Leavenworth, Kan., on fraud, money laundering and conspiracy charges.

He was convicted for his role in a scheme to take money from investors for the purported purchase of electronic goods for resale to big-box retailers. However, there were no goods and investors’ funds were used to finance other projects for Petters while funds from new investors were used to pay off old investors and make the operation look legitimate.