By Lynnley Browning

(Reuters) - Federal prosecutors in New York are building a criminal-fraud case against Liechtenstein's oldest bank, Liechtensteinische Landesbank, that could result in charges it helped wealthy Americans evade taxes, according to persons briefed on the matter.

These persons said an investigation has been under way for at least six months.

"We do not wish to comment on this matter," Cyrill Sele, a spokesman for Liechtensteinische Landesbank, said on Friday when asked about the investigation.

Carly Sullivan, a spokeswoman for the U.S. Attorney's Office in the Southern District of New York, said the office never confirmed or denied the existence of investigations and declined to comment.

The U.S. Justice Department has ramped up an investigation of Swiss and Swiss-style banks with private banking operations that have enabled U.S. clients to evade billions of dollars in U.S. taxes over the last 10 years or so.

While that broad investigation is focused on Swiss financial giant Credit Suisse AG <CSGN.VX>, which received a target letter from the Justice Department; London-based HSBC Holdings Plc <HSBA.L> and an array of smaller Swiss private and cantonal banks, it has widened to include banks in other countries as well, according to persons briefed on the matter.

U.S. officials are angry that the existence of the request was made public by two Swiss newspapers within days of the letter's receipt by Switzerland's Secretary of State for International Financial Matters, Michael Ambuehl, and the Swiss point-man for negotiating the showdown with the Justice Department, according to these persons.

TAX HAVEN

The scrutiny of Liechtensteinische Landesbank turns a spotlight on the tiny Alpine principality of Liechtenstein, a noted tax haven with Swiss-style secrecy laws. Liechtenstein banks, trust companies and other intermediaries have figured in the background of indictments and criminal information charges against U.S. clients and Swiss bankers at UBS AG <UBS.N>, Credit Suisse and HSBC, according to court papers.

While Liechtensteinische Landesbank has not been named in any of those legal actions, it allegedly has been involved in offshore private banking schemes used for tax evasion, according to these persons.

In 2008, Mario Staggl, an executive at New Haven Trust company in Liechtenstein, was co-indicted with Bradley Birkenfeld, the former UBS private banker whose disclosures about tax evasion services at UBS resulted in the bank paying a $780 million fine, admitting to criminal wrongdoing and turning over the names of 4,450 of American clients -- a watershed in Swiss bank secrecy. A U.S. Senate subcommittee report in 2008 detailed how a large Liechtenstein bank, LGT, owned by the country's royal family, sold unreported private banking services to wealthy American billionaires.

American officials learned of Liechtensteinische Landesbank's identity and alleged role through scores of voluntary disclosures made by wealthy Americans in recent years to the U.S. Internal Revenue Service. The disclosures, which offer reduced fines and penalties in exchange for coming forward with secret offshore accounts, require U.S. taxpayers to provide detailed information about the network of banks, trusts, shell corporations and intermediaries they have used.

The deadline for entering into the second voluntary disclosure was Friday.

The private bank division of Liechtensteinische Landesbank has 60 billion CHF in assets (about $68 billion at current rates). The bank has not lost money due to the crackdown on Swiss private banking and net new money inflows rose 1.1 percent as of end-June, to CHF 529 million (about $600 million), compared with the previous year.

(Reporting by Lynnley Browning in Hamden, Conn.; editing by Howard Goller and Andre Grenon)