Luke Macgregor / Reuters, file A HSBC bank logo is highlighted by the sun in London in this file photo taken March 1, 2010.

A "pervasively polluted" culture at HSBC allowed the bank to act as financier to clients moving shadowy funds from the world's most dangerous and secretive corners, including Mexico, Iran, Saudi Arabia and Syria, according to a scathing U.S. Senate report issued on Monday.

The report [link to PDF here] which comes ahead of a Senate hearing on Tuesday, said large amounts of Mexican drug money likely passed through the bank.

HSBC's U.S. division provided money and banking services to some banks in Saudi Arabia and Bangladesh believed to have helped fund al-Qaida and other terrorist groups, according to an Al-Jazeera story on the report.

While the big British bank's problems have been known for nearly a decade, the Senate probe detailed just how sweeping the problems have been, both at the bank and at the Office of the Comptroller of the Currency, a top U.S. bank regulator which the report said failed to properly monitor HSBC.

"The culture at HSBC was pervasively polluted for a long time," said Senator Carl Levin, chairman of the U.S. Senate Permanent Subcommittee on Investigations, a Congressional watchdog panel.

The report comes at a troubling time for a banking industry reeling from a multi-country probe into the manipulation of global benchmark rates. Last month, rival British bank Barclays agreed to pay a $453 million fine to settle a U.S.-British probe into the rigging of the benchmark interest rate known as the London interbank offered rate, or Libor.

Lax controls

The report caps a year-long inquiry that included a review of 1.4 million documents and interviews with 75 HSBC officials and bank regulators. It will be the focus of a hearing on Tuesday at which HSBC and OCC officials are scheduled to testify.

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In a statement emailed to NBCNews.com, the bank said:

We will apologize, acknowledge these mistakes, answer for our actions and give our absolute commitment to fixing what went wrong. We believe that this case history will provide important lessons for the whole industry in seeking to prevent illicit actors entering the global financial system.

The report also contained strong criticism of the OCC, saying the regulator failed to crack down on the bank despite multiple red flags, allowing money laundering issues "to accumulate into a massive problem".

The failings and lax controls inside HSBC included an inability to properly monitor $15 billion in bulk cash transactions between mid-2006 and mid-2009, inadequate staffing and high turnover in the bank's compliance units, the report said.

HSBC ignored risks in doing business in countries such as Mexico, a country rife with drug trafficking, it said.

Between 2007 and 2008, HSBC's Mexican operations moved $7 billion into the bank's U.S. operations. According to the report, both Mexican and U.S. authorities warned HSBC that the amount of money could only have reached such a level if it was tied to illegal narcotics proceeds.

The focus of the Senate probe was HSBC's U.S. operations, which has its main office in New York. HSBC used the U.S. unit as a selling point to clients outside the United States, touting its ability to handle U.S. dollar transactions.

Red flags

The report described that among HSBC's problems was the bank's compliance division being unable to battle the suspect money. High turnover of top compliance officials made it difficult for reform to take hold, the report said. Employees were "overwhelmed" by a mounting number of suspect transactions that needed review.

HSBC, according to the report, helped move money for a Mexican foreign-exchange dealer called Casa de Cambio Puebla that served as a hub for laundered proceeds, according to the report.

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Between 2005 and 2007, there was a "growing flood" of U.S. dollars moving between the exchange house and HSBC, setting off red flags inside HSBC. Some bankers said the transfers were legal. One said the money came from Mexican landscapers working in the United States and routing money back home to their families.

HSBC ultimately closed the account in November 2007 after it received a seizure warrant from the Mexican attorney general seeking money tied to the exchange dealer, the Senate report said.

Some of the money that moved through HSBC was tied to Iran, the report said, which would violate U.S. prohibitions on transactions linked to it and other sanctioned countries.

Between 2001 and 2007, more than 28,000 transactions were identified by an outside auditor for HSBC that potentially could have run afoul of laws that prohibit transactions with sanctioned countries. Of those, 25,000 involved Iran. A smaller number required additional analysis to determine if violations of U.S. regulations had occurred, the report said.

In 2010, Wachovia agreed to pay $160 million as part of a Justice Department probe that examined Mexican transactions, according to a BBC report, which also said ING last month agreed to pay $619 million to settle U.S. government allegations that it violated U.S. sanctions against Cuba and Iran.

Reuters contributed to this report.

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