An FBI spycatcher was accused yesterday of helping to send American agents to their deaths by trading secrets to Russia for $1.4m, in one of the biggest security breaches in United States history.

Robert Hanssen, a 56-year-old FBI veteran and father of six, faces charges stretching back more than 15 years, contained in a 110-page affidavit that details payments to Swiss bank accounts and in diamonds.

They include identifying former Soviet KGB double-agents who were working for the US.

“I think one of the worst parts is agents killed in the line of duty,” said Louis Freeh, the FBI’s director, who described the arrest as a coup for the bureau. “This case is a grave and tragic moment for us.”

Asked if the case was one of the worst breaches of US security, Mr Freeh replied: “I would characterise it in that fashion.”

Mr Hanssen, who has worked for the FBI for 27 years, was said to have been caught red-handed three days ago while leaving classified information at a “dead drop” in a park in northern Virginia. A payment of $50,000 was found nearby.

FBI agents said Mr Hanssen helped to cross-check and confirm intelligence sold by Aldrich Ames, thought to be the most damaging mole in CIA history. Ames, who was arrested six years ago, is serving a life sentence for revealing more than 100 operations and betraying about 30 agents, 10 of whom were executed.

Mr Hanssen, who is said to have offered his services to the former Soviet Union in 1985, gave details about KGB double-agents who had already been compromised by Ames.

“When these two KGB officials returned to Moscow they were tried and executed,” said Mr Freeh. “Two were executed and one was in prison and later released.”

John Ashcroft, the US attorney general, said: “The arrest should remind us that our free society is an international target in a dangerous world.”

Mr Hanssen, known to his Russian handlers by the code-name Ramon, is accused of passing on secrets about US electronic surveillance techniques. The four handlers did not know where he worked because he was able to use his experience to prevent both sides from following him.

High security clearance and sensitive assignments gave Ramon, who is thought to have been operating alone, access to details of US security operations. In the end, KGB documents that fell into the hands of FBI-CIA officers investigating a mole in their ranks led them to Mr Hanssen, who was due to retire in two months.

While he was supposed to have been operating as a counter-espionage officer spying on the spies in New York and Washington he was actually working for the Soviet Union and then Russia.

“In short, the insider betrayed his trust,” Mr Freeh said. “The FBI entrusted him with some of its most sensitive matters and the US government relied upon him for his service and integrity. The crimes alleged are an affront not only to his fellow FBI employees but to the American people.”

The Russian embassy in Washington declined to comment, but a security services spokesman in Moscow suggested that publicity about the case was politically motivated.

Plato Cacheris, a prominent Washington lawyer who is representing Mr Hanssen, said: “He’s quite upset, emotional. It’s a serious matter, an FBI agent charged with espionage.”

Mr Cacheris, whose previous clients have included Ames and Monica Lewinsky, spoke after Mr Hanssen appeared in court in Alexandria, Virginia, dressed in a black shirt and grey trousers. “We’ll have to see what the quality of the case is. As far as I know he is of good character,” Mr Cacheris said.

He said that Mr Hanssen, who has been working recently for the state department and is only the third FBI agent to have been accused of espionage, intends to plead not guilty. Mr Hanssen could face the death penalty if he is convicted.

Nancy Cullen, a neighbour of the Hanssen family, who live in a $300,000 (£200,000) split-level brick and cedar home in a Washington suburb, said: “We just can’t believe it… a dog and six kids and a regular routine.

“Part of the routine that made us feel guilty was that every Sunday they would pack up the van and go to church. They’re just a great family.” Mr Hanssen’s wife Bonnie teaches religious education part-time at a Catholic school.

“They didn’t appear to be unusual in any way,” said another neighbour. “They did nothing to raise a red flag or make me suspicious.”