LONDON, UK — Mahdi Hashi was a 19-year-old community worker from north London when he left Britain for Somalia to care for his ailing grandmother in 2009.

He kept in close contact with his family back in London until last summer, when he vanished.

When he resurfaced months later, he was in US custody. And he was no longer a UK citizen.

Strange, but not unique: Hashi is one of at least 16 people the British government has quietly stripped of their citizenship on the grounds of national security since 2010.

The practice is raising questions about government secrecy just as revelations from the American whistleblower Edward Snowden are drawing attention to security policies in the United States and Europe.

British law allows the Home Secretary to withdraw citizenship from those holding a second nationality whenever it’s deemed “conducive to the public good,” in the words of the Home Office.

The law — in place since 2002 but used regularly only since the current government took power in 2010 — was first uncovered in February by the Bureau of Investigative Journalism, a London-based NGO.

Its targets don’t have to have lived in the countries of their second nationality, or even be very familiar with them. At least five of those who lost their citizenship were born in Britain.

Once decided, the revocation is effective immediately, with no notice apart from a letter sent to the subject’s most recent address stating that he or she is no longer a citizen.

In most cases, the subjects lost their citizenship while abroad. Many learned their British passports were no longer valid and that they could not return home only when contacted by relatives back home who received notification letters.

Once the British government washes its hands of former citizens, it’s no longer responsible for anything that happens to them.

Two men stripped of their British nationality in 2010 were subsequently killed in US drone strikes in Somalia in 2012.

At least one other — Mahdi Hashi — was taken into US custody immediately after losing his citizenship.

Critics say such developments go against the spirit of British law.

“The government is acquiescing to actions by the American government that it itself would consider illegal,” said Asim Qureshi of CagePrisoners, a London-based campaign group that works with those deprived of citizenship and others affected by the war on terror.

The Home Office declined to comment on any individual case or elaborate about when it decides to act.

“The Home Secretary has the power to remove citizenship from individuals where she considers it is conducive to the public good,” a Home Office spokesman said in a statement. “An individual subject to deprivation can appeal to the courts.”

Stripped of their passports and unable to re-enter the country, however, would-be appellants have found it virtually impossible to challenge the rulings against them.

One man known in court documents as L1 who fled Sudan as a refugee in 1991 was naturalized as British in 2003. Home Secretary Theresa May revoked his citizenship in 2010 days after he left for a summer vacation with his family.

By the time L1 learned he was no longer a British citizen, the 28-day appeals period had expired.

Criticizing such actions in a letter to the UN Office of the High Commissioner for Human Rights, the Immigration Law Practitioners’ Association — which advocates on behalf of immigrants — wrote that “the individual deprived of his/her nationality is outside the jurisdiction, with no possibility of return, and there is a potent cocktail for a procedure that is anything but effective and can properly be characterized as arbitrary.”

Mahdi Hashi’s case is particularly complicated.

Born in Somalia, he grew up in London after emigrating as a toddler. Active in community outreach programs, he worked as a caretaker to an elderly man. Friends interviewed by various media denied he was ever involved with extremist groups in the UK.

“His life was very much geared toward being involved in social justice issues and being part of the community,” Qureshi of CagePrisoners said. “That’s why so much of this comes as a big surprise in terms of the extent of the harassment that he faced.”

In 2009, Hashi was part of a group of young Muslim men that accused the British security agency MI5 of harassing them and pressuring them to spy on its behalf.

He said security officials had detained and interrogated him on multiple occasions when he had attempted to travel abroad.

The first time he tried to travel to Somalia to visit his grandmother, in 2009, he was stopped in transit in Djibouti and sent back to Britain. Upon his arrival at Heathrow, he said, he was questioned by plainclothes officers who told him they would lift his travel restrictions if he agreed to work with MI5.

“I told him, ‘This is blatant blackmail,’” the Independent newspaper quoted Hashi as saying in 2009. “I looked at him and said 'I don't ever want to see you or hear from you again. You've ruined my holiday, upset my family, and you nearly gave my sick grandmother in Somalia a heart attack.’"

Later that year, Hashi moved to Somalia, where he met his wife. The couple had a son in February 2012.

After he disappeared last summer, his family received a letter stating that he was no longer a British citizen.

Soon after, a former detainee of a US-run detention camp in the east African country of Djibouti sent word that Hashi was being held there, although it was unclear how or why he ended up there.

The man said Hashi was being interrogated by the US authorities and had been asking to see a representative of the British government, not knowing it no longer recognized him as a subject.

The Foreign Office rebuffed the family’s appeals for help.

Last December, the FBI charged Hashi and two other men in a Brooklyn federal court with providing material support to the Somalia-based al-Qaeda affiliate al-Shabaab.

FBI Assistant Director George Venizelos said in a statement that the men had taken part in “terrorist operations” with the militant group. “As alleged, these defendants are not aspiring terrorists, they are terrorists,” it read. “Their capture and prosecution are important steps in the continuing campaign against terrorism.”

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It’s still not clear how or why the case landed in the US justice system. A lawyer for one of the other defendants told Reuters it was the first such case he’d seen against foreign militants with no obvious connection or clear threat to US interests.

Hashi denies all charges against him through his American lawyer.

With the case still pending, Qureshi says CagePrisoners has no evidence about whether Hashi did or did not join al-Shabaab during his time in Somalia. But the kind of official harassment Hashi claims he suffered propels some toward extremism, he adds.

“It’s exactly this type of alienation, this type of harassment and abuse, that drives people from thinking they can take part in mainstream society,” he said. “Often people find themselves in situations where they feel like they have nothing else."