IRS agents thought it was unusual when 90 percent of accountant Robert Trauffer’s clients – city police and firefighters – were getting refunds annually from 2001-2004.

So, in 2005, agents opened an investigation of Trauffer, then Frankford Hospital’s business director, who was doing income-tax returns on the side, and began auditing 34 returns over four years of cops and firefighters.

“Word traveled through the departments that you could get a refund” if cops and firefighters used Trauffer as an accountant, said Assistant U.S. Attorney Michelle Morgan-Kelly. “But then, they had to pay the refunds back, which was humiliating and infuriating to them.”

Yesterday, the bill came due for Trauffer before U.S. District Judge Cynthia M. Rufe, who sentenced him to 10 months in prison, three years of supervised release during which he is not allowed to do income-tax returns, restitution to the IRS, which he already had paid, and a $38,000 fine and $3,800 for special assessment.

Trauffer told Rufe that he had been acting out of greed when he filed the phony returns.

He apologized to the IRS, to his family and to his clients, and acknowledged the seriousness of the charges.

Rufe said she didn’t believe that Trauffer would ever be back in court, but decided that some period of incarceration was necessary to send a message to the community.

About 15 family members and about 35 clients attended the sentencing.

Trauffer became aware of the IRS investigation in 2005 and was indicted in March 2008 on four counts of income-tax evasion and 34 counts of filing false income-tax returns between 2001-2004.

Six months before he signed a plea agreement in November 2008, he repaid the IRS $351,000 for back taxes, interest and penalties, said his attorney, Michael Engle.

Trauffer pleaded guilty last February, but, due to legal technicalities, his sentencing was not until yesterday.

“He made sure not one of his clients was out of any money,” said Engle. “Many of his clients still use him as a tax preparer.”

Trauffer voluntarily paid $52,465 to either the IRS for clients who had not repaid their IRS refunds, or to clients who had repaid their refunds.

“He had an extreme fear of going to prison,” said Morgan-Kelly, who sought an 18- to 24-month prison term. “I suspect he paid the illegally obtained refunds back to his clients because he thought it would have an impact on the court.”

Engle told the judge that Trauffer had “made great efforts to turn his life around” by revamping his accounting practices and engaging in several charitable causes.

Among them were working with agencies that provide cancer victims with support, that help injured police and firefighters and that run a camp for underprivileged children. He also served on the community board of the Salvation Army.

Trauffer will surrender to the Bureau of Prisons in 30 days.