JAKARTA, Indonesia — The suicide attackers who struck the Indonesian capital last week planted a third bomb intended to send panicked crowds to hotel lobbies where the other bombs would explode, but the device’s timer malfunctioned, police said today.

The tactic, similar to that used by Jemaah Islamiyah terrorists in the Bali bombings, indicates that the attacks were intended to kill many more than the seven who died in the twin bombings at the two American-owned luxury hotels.

The unexploded device — a laptop computer filled with explosives and bolts — was found on the 18th floor of the J.W. Marriott hotel where the bombers had been staying and should have gone off first, said Ketut Untung Yoga of the national police.

“It is clear that the bomb found inside the hotel was equipped with a timer that shows the time of the (failed) explosion,” Untung Yoga said. “It was supposed to explode before the other two.”

Last today’s near simultaneous explosions at the Marriott and Ritz-Carlton killed seven and wounded more than 50, breaking a nearly four-year lull in terrorist activity in the world’s most populous Muslim-majority nation.

The two bombers, believed to have been associated with the regional terrorist network Jemaah Islamiyah, also died.

An unknown number of suspects have been picked up in a nationwide manhunt that is also targeting Malaysian fugitive Noordin Mohammed Top, the alleged mastermind of four major bombings in Indonesia.

Jemaah Islamiyah used a combination of stationary, timed explosives and suicide bombers in the 2002 and 2005 Bali bombings that killed more than 220 people. The group was also blamed for the first bombing of the J.W. Marriott in 2003 and an attack on the Australian Embassy in 2004.

A widespread crackdown by counterterrorism forces has netted hundreds of militants in recent years in Indonesia, and the group was believed to have virtually wiped out.

But last today’s attack showed that terrorists, possibly Noordin’s violent Jemaah Islamiyah splinter group, are still able to strike closely guarded Western targets in the heart of the capital, reviving fears that more bombings may follow.

Among those rounded up by police in central Java are Noordin’s wife, their two children and a broom-maker who allegedly confessed to police that he was trained to be a suicide bomber by Jemaah Islamiyah.

No one has been charged, but under Indonesian law they can be held for up to week for questioning.

Ariana Rahma, Noordin’s wife, told investigators she hadn’t known her husband’s true identity until his photo was shown on TV and in newspapers last weekend, said her lawyer, Achmad Kholid.

She believed that the man she married in 2005 — who was introduced to her by her father — was named Ade Abdul Halim and came from Sulawesi. Rahma last saw her husband and father June 22, her lawyer said, when the men narrowly escaped a police raid that found bomb-making material at the family home in the town of Cilacap.

She “learned from media reports that she is the wife of the most-wanted Islamic militant suspect in Southeast Asia,” Kholid said. “She told us that her husband and father decided to not return home because they were afraid to be captured.”