ANKARA, Turkey — The suicide bomber who struck the U.S. Embassy in Ankara spent several years in prison on terrorism charges but was released on probation after being diagnosed with a hunger strike-related brain disorder, officials said Saturday.

The bomber, identified as 40-year-old leftist militant Ecevit Sanli, killed himself and a Turkish security guard on Friday, in what U.S. officials said was a terrorist attack. Sanli was armed with enough TNT to blow up a two-story building and also detonated a hand grenade, officials said.

Turkish Prime Minister Recep Tayyip Erdogan said Friday that police believe the bomber was connected his nation’s  outlawed leftist militant group Revolutionary People’s Liberation Party-Front, or DHKP-C.  And on Saturday DHKP-C claimed responsibility for the attack in a statement posted on a website linked to the group. It said Sanli carried out the act of “self-sacrifice” on behalf of the group.

The group called itself “immortal” and said, “Down with imperialism and the collaborating oligarchy.” But it gave no reason for attacking the U.S. Embassy.  The authenticity of the website was confirmed by a government terrorism expert who spoke on condition of anonymity in line with rules that bar government employees from speaking to reporters without prior authorization.

Turkey’s private NTV television, meanwhile, said police detained three people on Saturday who may be connected to the U.S. Embassy attack during operations in Ankara and Istanbul. Two of the suspects were being questioned by police in Ankara, while the third was taken into custody in Istanbul and was being brought to Ankara.

NTV, citing unidentified security sources, said one of the suspects is a man whose identity Sanli allegedly used to enter Turkey illegally, while the second was suspected of forging identity papers. There was no information about the third suspect.

Earlier, Turkish Interior Minister Muammer Guler said Sanli had fled Turkey after he was released from jail in 2001, but managed to return to the country “illegally,” using a fake ID. It was not clear how long before the attack he had returned to Turkey.

NTV said he is believed to have come to Turkey from Germany, crossing into Turkey from Greece. Police officials in Ankara could not immediately be reached for comment.

DHKP-C has claimed responsibility for assassinations and bombings since the 1970s, but it has been relatively quiet in recent years. Compared to al-Qaida, it has not been seen as a strong terrorist threat.

Sanli’s motives remained unclear. But some Turkish government officials have linked the attack to the arrest last month of dozens of suspected members of the DHKP-C group in a nationwide sweep.

Speculation also has abounded that the bombing was related to the perceived support of the U.S. for Turkey’s harsh criticism of the regime in Syria, whose brutal civil war has forced tens of thousands of Syrian refugees to seek shelter in Turkey. But Prime Minister Erdogan has denied that.

Officials said Sanli was arrested in 1997 for alleged involvement in attacks on Istanbul’s police headquarters and a military guesthouse, and jailed on charges of membership in the DHKP-C group.

While in prison awaiting trial, he took part in a major hunger strike that led to the deaths of dozens of inmates, according to a statement from the Ankara governor’s office. The protesters opposed a maximum-security system in which prisoners were held in small cells instead of large wards.

Sanli was diagnosed with Wernicke-Korsakoff syndrome and released on probation in 2001, following the introduction of legislation that allowed hunger strikers with the disorder to get appropriate treatment. The syndrome is a malnutrition-related brain illness that affects vision, muscle coordination and memory, and that can cause hallucinations.

Sanli fled Turkey after his release and was wanted by Turkish authorities. He was convicted in absentia in 2002 for belonging to a terrorist group and attempting to overthrow the government.

On Saturday, the U.S. flag at the embassy in Ankara flew at half-staff and already tight security was increased. Police sealed off a street in front of the security checkpoint where the explosion knocked a door off its hinges and littered the road with debris. Police vehicles were parked in streets surrounding the building.

The Ankara governor’s office, citing the findings of a bomb squad that inspected the site, said Sanli had used 6 kilograms (13.2 pounds) of TNT for the suicide attack and also detonated a hand grenade.  That amount of TNT can demolish “a two-story reinforced building,” according to Nihat Ali Ozcan, a terrorism expert at the Ankara-based Economic Policy Research Foundation of Turkey.

Officials had earlier said that the bomber detonated a suicide vest at the checkpoint on the outer perimeter of the compound.

The guard who was killed was standing outside the checkpoint. The U.S. ambassador on Saturday attended his funeral in a town just outside of Ankara.

A Turkish TV journalist was seriously wounded and two other guards had lighter wounds.

DHKP-C’s forerunner, Devrimci Sol, or Revolutionary Left, was formed in 1978 as a Marxist group openly opposed to the United States and NATO. It has attacked Turkish, U.S. and other foreign targets since then, including two U.S. military contractors and a U.S. Air Force officer.

The group, designated as a terrorist organization by the United States and other European allies, changed its name to DHKP-C in 1994.

Friday’s attack came as NATO deployed six Patriot anti-missile systems to protect its ally Turkey from a possible spillover from the civil war raging across the border in Syria. The U.S., Netherlands and Germany are each providing two Patriot batteries.

Ozcan, the terrorism expert, said the Syrian regime, which had backed terrorist groups in Turkey, including autonomy-seeking Kurdish rebels, during the Cold War era and through the 1990s, had recently revived ties with these groups.

As Turkey began to support the Syrian opposition, Syrian President Bashar Assad’s regime began to try “rebuilding its ties with these organizations,” Ozcan said.

Radikal newspaper reported that the DHKP-C had recently been taking an interest in “regional issues,” reviving its anti-American stance and taking on “a more pro-Assad position.”

Former U.S. ambassador to Turkey Ross Wilson speculated that the masterminds of the embassy bombing may have been partly motivated by U.S.-Turkish policy on Syria.

“A successful attack would embarrass the Turkish government and security forces, and it would have struck at the United States, which is widely — if wrongly — thought to have manipulated the Erdogan government into breaking with Bashar al-Assad and supporting efforts to remove him from power,” Wilson, director of the Dinu Patriciu Eurasia Center at the Washington-based Atlantic Council, wrote in an analysis.  “That might rekindle public support for the group.  Alas for DHPK/C, this seems unlikely.”

Howard Eissenstat, a Turkey expert at St. Lawrence University in the United States, said the bombing showed that a “relatively isolated and obscure group” still has the capacity to cause havoc.

“They really fall outside of our comfortable narratives,” Eissenstat wrote in an email to The Associated Press. “And they do seem to have been left in an ideological time warp. There is something distinctly cult-like about them.”

The attack drew quick condemnation from Turkey, the U.S., Britain and other nations, and officials from both Turkey and the U.S. pledged to work together to fight terrorism.

It was the second deadly assault on a U.S. diplomatic post in five months.

On Sept. 11, 2012, terrorists attacked a U.S. mission in Benghazi, Libya, killing U.S. Ambassador Chris Stevens and three other Americans. The attackers in Libya were suspected to have ties to Islamist extremists, and one is in custody in Egypt.

U.S. diplomatic facilities in Turkey have been targeted previously by terrorists. In 2008, an attack blamed on al-Qaida-affiliated militants outside the U.S. Consulate in Istanbul left three assailants and three policemen dead.

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Associated Press writers Ezgi Akin and Burhan Ozbilici and Christopher Torchia in Istanbul contributed to this report.