The underground world of computer hackers has been so thoroughly infiltrated in the US by the FBI and secret service that it is now riddled with paranoia and mistrust, with an estimated one in four hackers secretly informing on their peers, a Guardian investigation has established.

Cyber policing units have had such success in forcing online criminals to co-operate with their investigations through the threat of long prison sentences that they have managed to create an army of informants deep inside the hacking community.

In some cases, popular illegal forums used by cyber criminals as marketplaces for stolen identities and credit card numbers have been run by hacker turncoats acting as FBI moles. In others, undercover FBI agents posing as "carders" – hackers specialising in ID theft – have themselves taken over the management of crime forums, using the intelligence gathered to put dozens of people behind bars.

So ubiquitous has the FBI informant network become that Eric Corley, who publishes the hacker quarterly, 2600, has estimated that 25% of hackers in the US may have been recruited by the federal authorities to be their eyes and ears. "Owing to the harsh penalties involved and the relative inexperience with the law that many hackers have, they are rather susceptible to intimidation," Corley told the Guardian.

"It makes for very tense relationships," said John Young, who runs Cryptome, a website depository for secret documents along the lines of WikiLeaks. "There are dozens and dozens of hackers who have been shopped by people they thought they trusted."

The best-known example of the phenomenon is Adrian Lamo, a convicted hacker who turned informant on Bradley Manning, who is suspected of passing secret documents to WikiLeaks. Manning had entered into a prolonged instant messaging conversation with Lamo, whom he trusted and asked for advice. Lamo repaid that trust by promptly handing over the 23-year-old intelligence specialist to the military authorities. Manning has now been in custody for more than a year.

For acting as he did, Lamo has earned himself the sobriquet of Judas and the "world's most hated hacker", though he has insisted that he acted out of concern for those he believed could be harmed or even killed by the WikiLeaks publication of thousands of US diplomatic cables.

"Obviously it's been much worse for him but it's certainly been no picnic for me," Lamo has said. "He followed his conscience, and I followed mine."

The latest challenge for the FBI in terms of domestic US breaches are the anarchistic co-operatives of "hacktivists" that have launched several high-profile cyber-attacks in recent months designed to make a statement. In the most recent case a group calling itself Lulz Security launched an audacious raid on the FBI's own linked organisation InfraGard. The raid, which was a blatant two fingers up at the agency, was said to have been a response to news that the Pentagon was poised to declare foreign cyber-attacks an act of war.

Lulz Security shares qualities with the hacktivist group Anonymous that has launched attacks against companies including Visa and MasterCard as a protest against their decision to block donations to WikiLeaks. While Lulz Security is so recent a phenomenon that the FBI has yet to get a handle on it, Anonymous is already under pressure from the agency. There were raids on 40 addresses in the US and five in the UK in January, and a grand jury has been hearing evidence against the group in California at the start of a possible federal prosecution.

Kevin Poulsen, senior editor at Wired magazine, believes the collective is classically vulnerable to infiltration and disruption. "We have already begun to see Anonymous members attack each other and out each other's IP addresses. That's the first step towards being susceptible to the FBI."

Barrett Brown, who has acted as a spokesman for the otherwise secretive Anonymous, says it is fully aware of the FBI's interest. "The FBI are always there. They are always watching, always in the chatrooms. You don't know who is an informant and who isn't, and to that extent you are vulnerable."