LONDON (AP) -- A British computer hacker's decade-long struggle to avoid trial in the U.S. over alleged breaches of military and NASA networks ended in success Tuesday, as the U.K. government ruled he was unfit to face charges there.

Home Secretary Theresa May said she had blocked the U.S. request to extradite Gary McKinnon after medical experts concluded he was seriously depressed and that there was "a high risk of him ending his life."

The 46-year-old unemployed computer administrator, who has been diagnosed with Asperger's Syndrome, was accused of one of the largest ever breaches of military networks, carried out soon after the Sept. 11, 2001, attacks in the United States.

"He literally couldn't speak, he cried, then we hugged, then we cried again," his mother Janis Sharp said, describing the moment she and McKinnon learned of his reprieve.

Officials in Washington expressed disappointment at the outcome, and State Department spokeswoman Victoria Nuland said the decision meant McKinnon would not "face long overdue justice in the United States."

British prosecutors will now decide if he should face charges in the U.K.

WHAT DID THE HACKER CLAIM?

McKinnon insists that he was scouring sensitive U.S. computer networks in an attempt to uncover concealed evidence of extraterrestrial life. Known online by the handle SOLO, McKinnon also claimed to have been attempting to expose security weaknesses.

He described how in 2001 and 2002 he spent about a year attempting to crack U.S. military systems - spending up to eight hours a day at a computer in his girlfriend's aunt's house while drinking beer and smoking marijuana.

McKinnon has since claimed that his hacking uncovered photographic proof of alien spacecraft and the names and ranks of "non-terrestrial officers."

He had offered to plead guilty to a hacking charge in Britain in order to avoid extradition. Prosecutors turned him down, insisting the U.S. was the correct venue for a trial.

WHAT WAS THE U.S. GOVERNMENT'S CASE?

U.S. officials said McKinnon's hacking shut down the U.S. Army district responsible for protecting Washington, D.C., and caused about $900,000 worth of damage. He was also accused of clearing logs from computers at Naval Weapons Station Earle in northern New Jersey, which tracks the location and battle-readiness of U.S. Navy ships.

At the time of McKinnon's indictment, prosecutor Paul McNulty said he had pulled off "the biggest hack of military computers ever, at least ever detected."

Prosecutors rejected the hacker's claim to have been hunting for signs of UFOs, pointing to a note McKinnon left on an Army computer, in which he derided U.S. foreign policy as "akin to government-sponsored terrorism."

McKinnon was caught in 2002 when investigators traced software used in the attacks to his girlfriend's e-mail account.

WHAT DID MCKINNON'S DEFENDERS SAY?

Supporters of McKinnon had insisted that he was unfit to be extradited to the U.S., because he has suffered with depression and is a suicide risk. They had argued that McKinnon was an eccentric but harmless man who had no malicious intent. "Gary is a classic computer nerd," his lawyer Karen Todner told BBC radio Tuesday.

British Prime Minister David Cameron -- who had championed McKinnon's case since before taking power -- raised the issue repeatedly with U.S. President Barack Obama, most recently during talks in March.

"It was an incredibly brave decision to stand up to another nation," Sharp, McKinnon's mother, told reporters.

Lawmaker David Burrowes, who represents McKinnon's London neighborhood, said May should be credited with "saving my constituent Gary McKinnon's life." Burrowes had threatened to quit his junior government post if the extradition was approved.

WHAT IS THE IMPACT OF HIS CASE?

In light of the McKinnon case, the U.K. government said it would seek major changes to rules governing extradition between Britain and the U.S.

Lawyers have complained that under "fast track" extradition procedures introduced after the Sept. 11 attacks, the U.S. is not required to offer substantial proof of an allegation when seeking to extradite a suspect from Britain.

May said she will seek to grant British courts new powers to refuse U.S. extradition requests. If the alleged crime involved took place mainly in the U.K., a court could decide that a prosecution should take place in Britain, rather than the U.S.

But in Washington, a Department of Justice spokeswoman stressed that McKinnon's case was exceptional and does not set a precedent for future cases.

"Our extradition relationship with the United Kingdom remains strong," spokeswoman Rebekah Carmichael said.

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Associated Press writers Cassandra Vinograd and Sylvia Hui contributed to this report