JAKARTA, Indonesia, Aug. 6 — The terror attack at the J. W. Marriott Hotel that killed at least 16 people and wounded 150 on Tuesday was almost certainly the work of Jemaah Islamiyah, a group that is closely linked to Al Qaeda, American officials said today.

Most of the senior members of Jemaah Islamiyah, including its top operations officer and its most skilled bombmakers, were still at large even though about 50 members of the group have been arrested since the attack on a Bali nightclub 10 months ago, they said.

The blast at the Marriott illustrated what had been known for months: that Jemaah Islamiyah, a group intent on achieving an Islamic state in Indonesia and other parts of Southeast Asia, consisted of many independent cells, each with the capacity to mount attacks, the officials said.

Indonesia is the most populous Muslim country in the world, and its sprawling archipelago with 220 million people lends near-perfect cover for fugitive terrorists.

The operations officer of Jemaah Islamiyah, known as Hambali, is among the "top echelon" of Al Qaeda members wanted by the United States, an American official said. The bomb experts, Dulmatin and Azahari Husin, believed to have made the bomb for the Bali explosion, have been targets of searches by the Indonesian authorities for many months.

Another bombmaking expert, Fathur Rohman al-Ghozi, escaped from a jail in the Philippines last month.

Amrozi, a defendant in the Bali bombing trials and an accused member of Jemaah Islamiyah, is scheduled to appear in a Bali court on Thursday to hear the verdict in his case. There was some speculation that the Marriott attack was timed as a warning to the government not to impose the death penalty, which Amrozi could face if convicted.

Another defendant in the Bali bombings, Imam Samudra, told reporters as he left the courthouse in Bali today that he was "happy" about the Marriott attack.

A sense of normalcy returned to the center of Jakarta today as Indonesian officials said they would impose new security measures at major hotels and businesses. But precisely what these would be was not immediately clear.

The choice of the 33-story Marriott left little doubt about the intentions of the terrorists.

"In 20-20 hindsight, the Marriott was an obvious choice," said Ken Conboy, who heads a security company, Risk Management Advisory. "It had a reputation as an American brand, it flaunted its American connection."

Interrogations of Jemaah Islamiyah members who have been arrested showed that they gave considerable thought to their choice of target, and its effects, Mr. Conboy said.

The Marriott, a frequent site for events organized by Americans, was considered to have some of the best security in the capital.

Even so, the trunks and undercarriages of cars were not inspected before they drove up the driveway to the front door, and floor-to-ceiling windows of the coffee shop that bore the brunt of Tuesday's attack faced the driveway.

At this year's July 4 celebration organized by the United States Embassy, officials took extra security precautions, including preventing cars from entering the driveway and forcing guests to walk about 100 yards from the foot of the driveway to the front door.

A Jakarta police spokesman, Prasetyo, said that the Indonesian authorities had found information about a planned attack on the Marriott when the police raided a Jemaah Islamiyah hideout in Semerang in central Java last month. Nine accused members of the group were arrested in connection with the raid.

But the United States Ambassador, Ralph L. Boyce, dismissed the notion that there was prior knowledge. "We didn't have any information that the Marriott was targeted," Mr. Boyce said today.

Because of mounting information of possible new attacks by Jemaah Islamiyah, the United States Embassy has been at a "very high state of alert," and stiff warnings had been in place for Americans living in Indonesia, and for Americans visitors, he said.

Of particular concern were "soft targets," such as hotels and restaurants, where foreigners gather, he said.

The Australian Foreign Minister, Alexander Downer, took the unusual step of predicting a possible terrorist attack in Indonesia "in the next day or so," and on Aug. 17, when Indonesia's independence is celebrated. "That is a day when we think it's possible there could be a terrorist attack in the central Jakarta area," Mr. Downer said.

American officials said they preferred not to be so precise about possible future attacks but, instead, to stress precaution at all times.

The methods used in the Bali and Marriott attacks were similar, the head of the Jakarta criminal investigation department, Erwin Mappaseng, said today.

Both attacks involved a vehicle packed with explosives being driven to the site. The police said today that potassium chloride and TNT, both substances used in the Bali bomb, were found on the Marriott driveway where a sports utility vehicle packed with explosives blew up.

A suicide bomber appears to have been in the vehicle, Indonesian officials said. A suicide bomber was involved in the Bali attack.

A senior Australian official said today that he was not surprised by the Marriott blast. "Everyone has been waiting for another attack," he said. "This is no more and no less than a reminder that Jemaah Islamiyah is still there, has the capability and will use that capability from time to time."

But the Australian warned that the Jakarta attack showed a ratcheting up of the Islamic group's tactics. "Bali was rationalized by the expatriates as being a tourist place, of being Hindu," he said.

By attacking a Western institution in the heart of the capital, the group was trying to create fear in the expatriate community, the official said. By scaring foreigners, the militants would drive the country's struggling economy into deeper trouble, a goal that was consistent with their ideal of wanting to establish a radical Islamic state in a place where most of the population remains moderate in its religious beliefs.