A suspected double bomber on the FBI's most wanted list who vanished for 21 years is due in court this week to decide if he will be sent back to the United States to face trial.

The FBI believes Daniel Andreas San Diego has links to animal rights extremist groups and is their prime suspect for a series of bombings in the San Francisco Bay Area in 2003.

Former FBI agents have stated there were 'missed opportunities' to arrest the 47-year-old before he vanished, highlighting a suspected 'bomb-making factory' found in his abandoned car after a chaotic chase in California.

Last year, San Diego was discovered 5,000 miles away, residing in a cottage in north Wales.

San Diego, with a $250,000 bounty on his head, faces extradition hearings at Westminster Magistrates Court in London following accusations of maliciously using explosives.

Both bombings San Diego is linked to occurred in 2003, targeting firms involved with animal testing. One was at a biotechnology corporation in Emeryville, California, and another at a nutritional products company in Pleasanton.

FBI officials described San Diego as remarkably unremarkable, making it difficult to track him, especially as he went off their radar following a strategic fugitive operation.

The authorities believe he had support during his time on the run and that he was not capable of such evasion without assistance.

FBI Director Christopher Wray emphasized the bureau's commitment to bringing fugitives to justice, stating that they will continue their pursuit no matter how long it takes.