File photo of Deutsche Bank Chief Executive Josef Ackermann sitting in a courtroom in Duesseldorf, western Germany, November 3, 2006. A suspicious envelope sent to Deutsche Bank Chief Executive Josef Ackermann was a letter bomb capable of exploding, investigators said December 8, 2011. No one has so far claimed responsibility for the package, which was intercepted late on Wednesday. [Photo/Agencies]

FRANKFURT - A suspicious envelope sent to Deutsche Bank Chief Executive Josef Ackermann was a letter bomb capable of exploding, investigators said on Thursday.

No one has so far claimed responsibility for the package, which was intercepted late on Wednesday.

It raised fears that a wave of protest against the failures and excesses of bankers could turn more violent, and prompted police across Europe to warn banks to be extra vigilant.

Ackermann, 63, a Swiss who is the first non-German to head Germany's biggest bank, is one of the few senior managers in the country always surrounded by bodyguards.

"Initial investigations show that this was an operational letter bomb," the Criminal Investigations Office for the state of Hesse and Frankfurt prosecutors in a statement, adding that no further information was available.    

Frankfurt's offshoot of the Occupy protest movement, which is critical of banks and has been staging protests in New York, Washington, London and many other cities, said it had nothing to do with the attempted attack.

"We condemn any action that is linked to violence," said Frank Stegmaier, an activist in the Occupy Frankfurt group, which has been camping outside the European Central Bank towers in the German financial capital since mid-October.  

"Occupy has other ways of protesting," he added.

Security has been stepped up at Deutsche Bank offices around the world, banking sources said, while police elsewhere in Europe warned banks to be extra vigilant.

Two Greek commercial banks said they had already been operating under top security conditions after similar letter bomb incidents last year.

One banking source said that since 2006 every item of mail sent to members of Deutsche Bank's executive committee was put through a security check.

Deutsche Bank employees heading to work said they did not feel threatened.

"There are always people who think a solution would be to make someone pay, but as an employee, I do not feel threatened," Stefan Popp told Reuters Television.

European Union leaders were to meet in Brussels on Thursday and Friday to try to agree on a way out of a euro zone sovereign debt crisis that has triggered a wave of austerity programmes around Europe and prompted Germans to fret that they may have to foot the bill.

Ackermann is the highest paid chief executive of a German blue-chip company, earning 9 million euros ($12 million) in 2010. He is chairman of the Institute of International Finance, the bank lobbying group negotiating a private-sector contribution toward a multibillion euro bailout of Greece.  

Due to retire in May next year after over 10 years at the head of Deutsche, he is credited with transforming the bank into a "global champion", and has become associated with Wall Street-style bonuses and a shareholder-driven management style.

Last month, Ackermann was whistled and shouted at by Occupy Movement members during a speech in the city of Hamburg.  

A previous Deutsche Bank head, Alfred Herrhausen, was murdered in 1989 by leftist Red Army Faction guerrillas who blew up his car.

($1 = 0.7468 euros)

(Additional reporting by Sabine Wollrab, Ed Taylor, Reuters TV and Harry Papachristou in Athens; Writing by Madeline Chambers in Berlin, Editing by Mark Heinrich)