When Bradley Birkenfeld, a former UBS banker, told the U.S. about the Swiss bank’s scheme to help wealthy Americans hide their assets from the taxman, he was sentenced to 40 months in prison for his role in the scheme.

Given the government’s aggressive prosecution of the person who blew the whistle on illicit Swiss banking practices, why haven’t there been more prosecutions in the years since against the bankers and tax cheats who withheld enormous revenues from the U.S. government?

A bipartisan report released yesterday by the Senate Permanent Subcommittee on Investigations paints a picture of “lax enforcement” by U.S. authorities who have been investigating misconduct throughout the Swiss banking system in the wake of the UBS case.

“Despite the passage of five years” since the UBS scheme was exposed, “U.S. law enforcement failed to prosecute more than a dozen Swiss banks that facilitated U.S. tax evasion, failed to take legal action against thousands of U.S. persons whose names and hidden Swiss accounts were disclosed by UBS, and failed to utilize available U.S. legal means to obtain the names of tens of thousands of additional U.S. persons whose identities are still being concealed by the Swiss,” the report says.

In 2008 and 2009, the Subcommittee spotlighted offshore tax havens in general, with a particular focus on UBS’s scheme. Now the panel is slamming the Department of Justice (DOJ) for taking a passive approach to prosecuting another major Swiss bank, Credit Suisse, which was “deeply involved in facilitating U.S. tax evasion and whose unnamed U.S. customers continue to owe unpaid taxes on billions of dollars in hidden assets,” according to the report.

The Department defended its enforcement record in a written statement prepared for the Subcommittee, which held a hearing yesterday to accompany the release of its report. Since 2009—when DOJ reached a “ground-breaking deferred prosecution agreement with UBS” that included a $780 million fine and the disclosure of 4,700 accounts with the names of U.S. clients—the Department has “publicly charged 73 account holders and 35 bankers and advisors,” according to the statement.

DOJ officials also pointed to “on-going criminal investigations” that could lead to further enforcement actions.

But Subcommittee members from both sides of the aisle expressed dismay at the lack of prosecutions to date, as illustrated by the government’s treatment of Credit Suisse.

The bank engaged in “illicit banking practices” that “belong in a spy novel,” Subcommittee Ranking Member John McCain (R-AZ) said in an opening statement, citing “[r]emotely-controlled elevators leading to hidden rooms in the bank’s Zurich headquarters,” among other things. But while DOJ indicted a number of Credit Suisse bankers, it has “failed to prosecute those indictments,” McCain said.

Furthermore, while Credit Suisse had more than 22,000 U.S. customers with Swiss accounts in 2006—the vast majority of which were undeclared—the U.S. has obtained the names of only 238 customers due to restrictions imposed by the Swiss government. “To me, getting 238 in five years out of a universe of 22,000, less than one percent, is more than an embarrassment,” Subcommittee Chairman Carl Levin (D-MI) said in his opening statement.

The Subcommittee’s report concludes that DOJ’s “recent record of lax enforcement” is “puzzling in light of the massive tax revenues still owed and uncollected.” The Department’s record “not only fail[s] to reflect U.S. values favoring bank transparency and taxpayer honesty, but also set[s] a troubling precedent for how DOJ will approach other offshore banks around the world that facilitate U.S. tax evasion,” the report says.

The U.S. government’s posture towards Swiss bank secrecy and tax evasion is unfolding in a broader context where there are potentially competing interests, such as the government’s diplomatic relationship with Switzerland.

Whistleblower advocates say the government should be doing more to incentivize and protect tipsters who help to expose offshore tax evasion.

“While whistleblowers have given the federal government the roadmap on how to pursue illegal offshore accounts, too often the decision makers in DC have not been willing to pull over and ask for directions,” according to a letter sent to the Subcommittee by the National Whistleblowers Center, which represented Birkenfeld.

Last year, the Project On Government Oversight and the Government Accountability Project objected to a proposed rule that would, in the words of Senator Charles Grassley (R-IA), “hamstring” a government whistleblower program by “limiting whistleblower awards and discouraging knowledgeable insiders from coming forward.”