A Briton and others accused of running a massive international online child porn ring went to enormous lengths to keep investigators from discovering their website, US authorities said.

Members, who numbered 1,000 at its peak, were granted access to the hierarchical group's website by invitation only and had to be vetted by senior members. Only the most trusted members controlled the huge stash of child pornography and members advised one another in forums on how best to cover their tracks.

But a single weak link brought the network crashing down, allowing investigators to follow a trail of incriminating data from the website to its users and net more than 50 arrests so far.

US prosecutors are seeking the extradition of several suspects from overseas, including the group's alleged mastermind, Delwin Savigar, 35, of Leyland, Lancashire, who is serving a 14-year prison term in Britain for sex offences against three underage girls.

Savigar is said to have created and run the password-protected site. His lawyer did not immediately respond to a phone message seeking comment.

"Any time we can get into and, for lack of a better term, 'bust' an online community that has protected itself, that is a big accomplishment," said Matthew Dunn, section chief of the child exploitation section at the US Immigration and Customs Enforcement Cyber Crimes Centre, which took part in the investigation.

Prosecutors revealed details of their two-year investigation this week, saying more than 50 arrests had been made in the US, including 22 of 26 suspects who have been charged in Indianapolis in an alleged conspiracy to distribute child pornography.

Nineteen of the 22 defendants have pleaded guilty or been convicted and authorities are trying to identify the other four suspects, known only by their screen names.

Authorities became aware of the website in late 2007, after a man in Alabama who was the target of another investigation gave the US Postal Inspection Service permission to use his electronic accounts, Mr Dunn said.

Investigators took over the man's email and online identity, and used it to access the website.

"Once we're in, and we see the material, there's your investigation," Mr Dunn said. "You realise they are distributing child pornography."

Investigators followed clues to computers across the world, including to hard drives in Indiana, where authorities decided to prosecute the case as a conspiracy.

Mr Dunn said authorities seized the website in February 2008, but its operators, who occasionally switched internet service providers to avoid detection, put it back online.

But in September 2008 the website was shut down for good, as authorities across multiple time zones arrested suspects simultaneously. Savigar was arrested in that swoop, along with suspects in New York, Baltimore, Michigan and California.

Prosecutors waited until this week to announce the arrests because they did not want to tip off suspects. The latest person arrested in connection with the conspiracy in the US, Edward Oedewaldt, made his first court appearance before a federal magistrate in Indianapolis this week. He has pleaded not guilty.

Oedewaldt was indicted in September 2008 under his screen name, "Legend", but was not arrested until last month, when authorities finally determined who he was and picked him up in Louisiana, Mr Dunn said.

"He took a lot of steps to hide his identity," Mr Dunn said. He said Oedewaldt bragged online that he did not think he would be caught.

According to court documents, forensic experts sifted through hard drives, discovering folders filed with hundreds of thousands of illegal images. They used software programs to trace online activity, recovered deleted files, and looked at online chats in which they say suspects incriminated themselves.

Some defendants co-operated and informed on other group members or told authorities about other forums, court document show.

Several defendants had criminal records for sexually assaulting minors or producing child porn pictures or video, according to court documents. At least two alleged members discussed adopting children, according to the filings.

"We went after everyone we could identify," said Steve Grocki, a Department of Justice lawyer who worked on the case.