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The Associated Press
Indonesian Muslims who are on trial for their involvement in a fatal attack against followers of a minority Islamic sect earlier this year wait for the start of the hearing at a holding cell at a district court in Serang, Banten province, Indonesia, Thursday, July 28, 2011. The court has sentenced the men to up to six months in jail for their roles in the attack. (AP Photo)

JAKARTA, Indonesia — The ringleader of a frenzied mob attack that killed three members of a minority Muslim sect was sentenced Thursday to 5 1/2 months in prison in a ruling decried by critics as too lenient and a blow to religious freedom in Indonesia.

Ten others also were convicted of less serious charges of weapons possession in the February violence and sentenced to between three and six months behind bars.

Indonesia, a predominantly Muslim and secular nation of 240 million, has a long history of religious tolerance.

But an extremist fringe has grown more vocal in recent years and the government, which relies on the support of Islamic parties in Parliament, has been accused of caving in to them. The government in 2008 banned the activities of the sect and rights activists say the decision might have encouraged the violence against the group.

The attack on members of Ahmadiyah — considered deviant by many conservative Muslims here and abroad because they do not believe Muhammad was the final prophet — placed Indonesia under the international spotlight because it was captured on video and widely circulated on the Internet.

It provoked condemnation in Europe and the U.S.

The clip showed 1,500 people storming a house in Banten province on Feb. 6 with machetes, rocks, and clubs to stop sect followers from worshipping.

They beat three men to death and injured six others before setting cars and houses ablaze.

Though a policeman showed up at the scene, his cries of "stop" were drowned out by chants from the crowd on "Allahu Akbar!" or "God is Great!"

The 17-year-old ringleader, Dani bin Misra, wearing a black leather jacket and a white skull cap, is seen on the footage eagerly smashing the skull of one of the lifeless victims with a rock.

The Serang District Court found him and 10 others guilty of illegal possession of weapons and involvement in the attack. None was charged with murder or manslaughter and the maximum penalty for weapons possession is 5 1/2 years imprisonment.

"This verdict is an embarrassment to Indonesia," said Rafendi Djamin, the executive director of the Jakarta-based Human Rights Working Group, adding that it shows how a small, radical fringe can influence the judicial system.

Police did not conduct thorough investigations, he said, and prosecutors failed to call key eyewitnesses to the attack in the village of Cikeusik.

"This will do nothing to deter such attacks on minorities in the future and only threatens the democratic process in our country," Djamin said.

The U.S. Embassy in Jakarta issued a statement saying it was "disappointed by the disproportionately light sentences" handed down Thursday. It encouraged Indonesia "to defend its tradition of tolerance for all religions."

The Setara Institute for Peace and Democracy, a human rights group, said that attacks by religious hard-liners were steadily increasing.

It listed 64 incidents last year — up from 18 in 2009 and 17 the year before — ranging from physical abuse to preventing groups from performing prayers and burning houses of worship.

Ahmadiyah, said to have around 200,000 followers across the archipelagic nation, has increasingly been a target. But the February attack was by far the most brutal.