NEW YORK (Reuters) - The American public wants to see bankers’ heads on spikes for triggering a global financial crisis, but so far prosecutors and regulators have come up empty.

Federal and state authorities are struggling to hold banks accountable, but cases have proven to be extremely complicated and hard to win, if they get brought at all.

In recent days, investigators and prosecutors have renewed their efforts, focusing their attention on complicated repackaged debt known as collateralized debt obligations.

Criminal charges are contemplated. But winning these cases won’t be easy, in part because it will be difficult to prove criminal intent to defraud or deceive.

“A financial meltdown is not a crime,” said David Siegal, a former federal prosecutor in New York.

Nor is being stupid or greedy.

The government’s only criminal case against Wall Street executives linked to the crisis -- two former Bear Stearns hedge fund managers accused of lying to investors -- ended with an acquittal last November.

A criminal investigation of the former American International Group AIG.N executives who helped the insurer take on exposure to toxic subprime securities is said to be petering out, people familiar with the matter said. And it's not clear whether a criminal inquiry into the collapse of Lehman Brothers will prove any more fruitful.

Prosecutors and regulators appear to be on a new tack now: looking to see if the big banks that designed and sold all those toxic mortgage-linked securities either duped their investors or the powerful rating agencies that slapped a Triple A stamp of approval on the deals.

On Wednesday, New York Attorney General Andrew Cuomo issued subpoenas to eight banks, including Goldman Sachs Group Inc GS.N, Morgan Stanley MS.N, Citigroup Inc C.N and UBS UBSN.VX, launching a probe into whether they misled rating agencies, a source said.

ELEMENT OF POLITICS

Some say there is an element of politics to many of these cases.

“It’s politics in the broadest sense,” said Ed Grebeck, the chief executive of Tempus Advisors and an instructor at New York University. “We know that Congress and the regulators who missed the structured finance credit crunch are trying to make a name for themselves acting like they are going to take the next step to stop this from happening again.”

Cuomo is a hard-charging Democratic attorney general running for governor in New York. Many say he will score points with voters by launching a probe, even if the investigation is unlikely to be finished by the time voters go to the polls in November.

Federal investigators are also looking to take Wall Street to task.

A source said that U.S. federal prosecutors, working with securities regulators, are conducting a preliminary criminal probe into whether four banks misled investors about their roles in mortgage bond deals.

The banks under early-stage criminal scrutiny include JPMorgan Chase JPM.N and Deutsche Bank DBKGn.DE.

Last month, the U.S. Securities and Exchange Commission charged Goldman with civil fraud over its marketing of a subprime mortgage product. Criminal prosecutors are also looking into the matter.

But some wonder whether authorities are late to the game and are now going after simplistic cases in a bid to make headlines.

“There were far more broad, far more serious, far more egregious and far more clear cut and consequential problems than the SEC’s action against Goldman,” said Joshua Rosner of the research firm Graham Fisher & Co, adding that he expected some major Wall Street criminal cases were yet to come.

PROVING INTENT

Proving criminal conduct is no easy task. Prosecutors must show that Wall Street bankers and marketers knowingly committed fraud by failing to disclose the hidden dangers in mortgage-linked securities that some say were designed to fail.

By contrast, in a civil securities case, regulators often only need to show reckless conduct to prevail.

Cases against Wall Street executives can be difficult to prove to the satisfaction of a jury because of the mind-numbing volume of emails, prospectuses, and memos involved in documenting a case.

“It’s not like a shooting on the street,” said Siegal, the former federal prosecutor, who is now an attorney with Haynes and Boone. “It’s an enormous and potentially overwhelming task. Typically what you need is a whistle blower.”

The complexity of the cases means that they take a long time to put together. Other high-profile cases in the Enron collapse and the Galleon insider trading case have taken years of investigation but ultimately did end with criminal charges being filed.

In those cases, some of the key players rushed to sign plea agreements once the government had made its cases.

A TIME FOR REFLECTION

This time around, prosecutors may have been waiting for financial firms to stabilize before needling them.

“In the midst of the crisis everyone was interested in making sure that the system did not collapse,” Rosner said. “No one was spending time looking at what led to it.

“Now there’s this view we’ve stabilized and maybe come out of it. Cooler heads prevail and people are now reflective and reflecting on what transpired.”

As prosecutors dig in, they are focusing on cases that seem simple and winnable. But the downside of that approach is that regulators are missing the bigger picture by zeroing too closely on individual transactions, critics say.

Janet Tavakoli, president of Tavakoli Structured Finance Inc in Chicago and author of a book on synthetic CDOs, called the activity of big banks “malicious mischief,” and said regulators’ efforts to bring accountability have amounted to “publicity stunts that aim to miss.”

“It distracts the public from the real issue of the damage done to the U.S. economy for which the taxpayer ended up being on the hook to bail out banks,” Tavakoli said.