FORT LAUDERDALE, Fla. — A former banker described by prosecutors as the man most responsible for sparking a broad U.S. investigation into rich Americans' use of secret Swiss bank accounts to evade taxes was sentenced Friday to more than three years in federal prison.

Prosecutors gave Bradley Birkenfeld, a 44-year-old U.S. citizen, credit for voluntarily disclosing illegal tactics by Swiss banking giant UBS AG and others. But they said Birkenfeld initially refused to confess his own misconduct and hoped to collect a cash reward under U.S. whistleblower laws.

"He refused to disclose his own wrongdoing," said Kevin Downing, senior trial attorney in the U.S. Justice Department's tax division. "It's a major problem for us."

Prosecutors had recommended a 2-year sentence, half off the potential maximum of 5 years for Birkenfeld's guilty plea to a single fraud conspiracy charge. U.S. District Judge William Zloch decided on a sentence of three years and four months, coupled with a $30,000 fine and followed by three years' probation.

Birkenfeld's attorneys pleaded for only probation, and Birkenfeld himself told Zloch his only goal was to expose wrongdoing at UBS and the Swiss banking system as a whole. Birkenfeld first met on his own with Justice Department investigators in summer 2007, which Downing said began a probe that has roiled the world's banks and triggered numerous criminal tax-evasion prosecutions.

"Without Mr. Birkenfeld walking into the door of the Department of Justice in summer of 2007, I doubt this massive fraud scheme would have been discovered by the United States government," Downing said.

Partly because of Birkenfeld's disclosures, UBS this week agreed to disclose to the U.S. the names of 4,450 wealthy Americans suspected of hiding assets and dodging taxes in secret accounts. Three other Swiss bankers and a Swiss lawyer also are facing U.S. criminal charges; four U.S. clients of UBS have already pleaded guilty, and at least 150 more are under investigation.

Birkenfeld, a Swiss resident for 15 years, said he did not initially tell investigators about his own role in tax crimes because he felt constrained by Swiss law prohibiting disclosures about bank clients unless ordered to by a court. Birkenfeld also did not have immunity from prosecution.

"If I divulged any names without a subpoena, I would go to jail in Switzerland," Birkenfeld said. But Downing said Birkenfeld was given assurances he would not be prosecuted by the Swiss.

One of his wealthy clients, California real estate magnate Igor Olenicoff, pleaded guilty in October 2007 to tax charges and was sentenced to two years' probation plus payment of about $53 million in taxes and penalties.

Downing said Birkenfeld will continue to cooperate and could receive future sentence reductions based on that. Birkenfeld was ordered to report for his prison term Jan. 8.

The Washington-based National Whistleblowers Center, which advocates for people who provide insider evidence of wrongdoing at corporations and in government, issued a statement saying Birkenfeld's prison term might deter others from coming forward.

"It stuns me that the reward for a whistleblower who shined the light on extensive tax fraud and corruption is being sent to jail," said Dean Zerbe, special counsel at the center.