NIIGATA — To his neighbors, Lionel Dumont was a mystery.

When police and immigration officials asked about the Frenchman, Dumont’s landlord had no idea who he was, even though the landlord lived right across the street and had only 36 tenants in his apartment building.

“They showed me a black-and-white picture and asked if I remembered him,” Jubei Sato said. “I couldn’t place him at all. I don’t think I saw him once the whole three months he lived here. He blended right in, never caused any trouble. But I found out after he left that he’d only paid half his rent.”

Last week, Sato — and the rest of Japan — found out why authorities were interested in the 33-year-old starting a few months ago.

Dumont, according to police, may be the first al-Qaeda operative to have infiltrated Japan since the Sept. 11, 2001, attacks on the United States. And he did it with amazing impunity, entering on a faked passport and repeatedly leaving and re-entering the country before slipping out again for good a year later.

Dumont’s arrival in July 2002 should have raised red flags. He was put on an international wanted list in 1999 after escaping from a Sarajevo prison, where he was serving a 20-year sentence for the murder of a Bosnian policeman during a robbery. He was in Bosnia fighting alongside other Muslims.

Dumont also was convicted in absentia by a French court in 2001 for a string of violent crimes while he was a member of the Roubaix gang and was sentenced to life in prison.

Though he was arrested in December at a Munich, Germany, hotel, his case made headlines only last month when he was extradited to France.

Dumont, a convert to Islam, is now jailed in the northern French city of Douai, in the region where the Roubaix gang operated. He will be retried in keeping with French law on absentia convictions.

It is too early to say how he will plead, according to the office of his lawyer, Dominique Sapin, who was unavailable for comment. Dumont has yet to go before an investigating judge.

Why Dumont came to Japan remains a puzzle.

Scrambling to find answers, police last week raided several businesses in and around Tokyo and arrested five men allegedly contacted by Dumont after he left Japan. The five — three Bangladeshis, an Indian and a Mali national — were arrested for alleged immigration violations or the falsification of documents.

The Dumont case has prompted calls from the highest levels for heightened vigilance, including an order by Public Security Intelligence Agency chief Takashi Oizumi for authorities to act as if “Japan were at risk of being the target of a terror attack tomorrow.”

But what is most frightening about the Dumont case for many is the challenge it poses to a deeply held assumption — that while the post-9/11 world is a dangerous place, this island nation is too homogeneous or too isolated to be penetrated by foreign terrorists.

“Who would have expected a person like that to have come to Japan, and especially here of all places?” said housewife Sumiko Tsuji, one of Dumont’s neighbors. “I guess they can be anywhere.”

Niigata would seem an unlikely place for a foreign fugitive.

Located on the Sea of Japan coast, it is far from the anonymous crowds of Tokyo. Surrounded by rice paddies and lush green hills, it maintains the feel of a small, provincial capital despite its population of 480,000.

But a short drive to the eastern outskirts of town reveals a very different scene, where billboards more often than not are in Russian or English. In this neighborhood, called East Port, crowds of Muslims gather each day at a makeshift mosque in a prefabricated building on a hill not far from the docks. Used-car lots, most run by Pakistanis, stretch as far as the eye can see.

It is not a place many people go unless they have business. And much of the business done here has long been considered suspect — more than a few stolen cars are believed to make their way into the legitimate shipments headed for Russia, the Middle East and South America.

Unlike his low profile in town, Dumont was well-known here.

“We all called him Samir,” car dealer Nadeem Abdul said. “He was a gentleman. Very devout, a very hard worker.”

According to Abdul and several other dealers, Dumont worked at different car lots, parking and taking care of the vehicles.

“He didn’t drink or smoke or go to bars,” Abdul said. “He couldn’t make much money, but he seemed to be relaxed. His life was good.”

Good enough, Abdul said, that Dumont wanted to return.

“He called me about seven months ago and said he was working at a sushi restaurant, but he didn’t say where,” Abdul recalled. “He said he was hoping to come back again.”

Dumont is suspected of trying to establish an al-Qaeda cell to carry out a terrorist attack, Japanese media have reported, quoting anonymous police officials who called him a “senior al-Qaeda member.”

But French authorities say that is “largely exaggerated.” Police contacted by The Associated Press confirmed only that they believe Dumont was linked to al-Qaeda, but they refused to elaborate.

Prosecutor Joachim Ettenhofer, who handled the case in Europe, said the French extradition request cited only robbery accusations and did not mention terror-related activities. But French investigators have linked the Roubaix gang, named after the city where it was based in the mid-1990s, to a radical Islamic network, contending that robberies were used to finance extremist activities.

Ahmed Ressam, an Algerian convicted in the United States of planning to bomb the Los Angeles airport during millennium celebrations, was linked to the gang. A lack of solid evidence meant that terrorism-related charges were never brought against Dumont or other members.

Japanese police also reportedly suspect Dumont might have been laundering money, noting he made several trips to Europe and Asia before leaving Japan for Malaysia in September 2003.

Dumont might have sought refuge here because entry is relatively easy for Westerners. As the bearer of a French passport, the blue-eyed Dumont would not have been as closely scrutinized as a Middle Eastern visitor.