KUTA, Indonesia - Indonesian police said yesterday that they had arrested a man who admitted planting the car bomb that blew up a Bali nightclub in a terrorist attack last month.

The announcement by the national police chief, Dai Bachtiar, was described by a senior American official as the first promising sign in the investigation.

The arrest of the man - identified by the police as Amrozi, who they said had links to an extremist Islamic group - took place Tuesday in the town of Tenggulun in eastern Java, the main island of Indonesia, which abuts Bali, police said.

They said Amrozi was the owner of a L300 Mitsubishi minivan that was parked in front of the Sari club moments before the bomb exploded.

The arrest could turn out to be the result of some painstaking work by the head of the police investigation, Gen. I Made Mangku Pastika.

There have been more than a dozen arrests since the Oct. 12 blast, which killed more than 180 people.

Most of the victims were young Western tourists, many of them from Australia.

Amrozi, the first person to be named as a suspect, is in custody in Bali, the police said.

According to Indonesian television accounts, Amrozi was affiliated with an Islamic boarding school supported by the leading Indonesian extremist cleric, Abu Bakar Bashir. Followers of Bashir founded the school in Tenggulun and Bashir visited the school last year, the accounts said.

The police chief told reporters that Amrozi admitted owning a Mitsubishi van and using it to carry out the bombing in Bali.

The U.S. official said that it appeared Amrozi could be critical in buttressing information already given by an operative of al-Qaida who has described the inner workings of Jemaah Islamiyah, an Indonesian-based radical Islamic organization with links to al-Qaida. The operative, Omar al-Faruq, was arrested in Indonesia in June and has been in U.S. custody ever since.

Bashir, who was arrested by the Indonesian police two weeks ago, is considered the intellectual leader of Jemaah Islamiyah. The United States has said it believes Jemaah Islamiyah organized the Bali bombing. But Bashir has not been charged in connection with the Bali blast.

Jemaah Islamiyah, which was also involved in plotting attacks against U.S. embassies in Singapore and Malaysia last December, was declared a terrorist organization by the United States, as well as Indonesia, shortly after the Bali blast.

The head of the Islamic school where Amrozi was arrested said on an Indonesian television newscast today that Amrozi worked in Malaysia in the 1990s. Bashir lived in exile in Malaysia during that period because his desire to found an Islamic state in Indonesia brought him into conflict with the sectarian rule of the dictator Gen. Suharto.

Police said Amrozi resembled one of four sketches issued by the Indonesian authorities of suspects in the Bali bombing. A number of Indonesian men have been arrested since the publication of the sketches but have been released.

A Western diplomat said last night that there were reports from Indonesian law enforcement officials that a search of Amrozi's house had turned up compact discs containing material by Osama bin Laden, al-Qaida's founder.