BEIRUT, Lebanon — The murder was straight out of a cheap spy thriller. At least 11 professional assassins, some wearing wigs and fake beards, tracked a senior Hamas official to his Dubai hotel in January and killed him with cold precision, fleeing the country afterward on European passports, the Dubai police say.

But even as the Dubai authorities called Tuesday for an international manhunt, questions emerged about the identities of the suspects, deepening the mystery around the killing.

British and Irish officials said the suspects’ passports — which were unveiled at a news conference Monday by the Dubai police, along with their photographs and surveillance video — appeared to be fake, and in at least three cases appeared to have been stolen from British citizens living in Israel.

“We believe that the passports used were fraudulent and have begun our own investigation,” the Foreign Office in London said in a statement. Six of the 11 suspects identified by the Dubai police on Monday are British and three are Irish. In Dublin, the Department of Foreign Affairs said that it had been “unable to find any record of Irish passports having been issued with details corresponding to those published in Emirati newspapers,” and added that “we have received no evidence that any Irish nationals were involved.”

An Emirati official said the passports had been used repeatedly months before the killing, in Europe and Asia. He added that the hit team had included a total of 17 people, six of whom had not yet been identified.

In Israel, a British man named Melvyn Adam Mildiner told Reuters that he had the same name as one suspect, but that he was a different person from the one whose photograph was provided by the Dubai police, and that he had his passport with him.

“I am obviously angry, upset and scared — any number of things,” Mr. Mildiner was quoted as saying. “And I’m looking into what I can do to try to sort things out and clear my name.”

Two other British men living in Israel — Stephen Daniel Hodes and Paul John Keeley — also appear to have had their identities used by the suspects, according to Israel’s Channel Two News, which interviewed the men.

Because the victim, Mahmoud al-Mabhouh, was a senior Hamas official, many have suspected that Israel was behind his assassination. Hamas has accused Israel and vowed revenge. Israeli officials have not confirmed or denied the Hamas accusations.

The Dubai police chief, Dahi Khalfan al-Tamim, did not accuse Israel, but said it was possible that a foreign government had ordered the killing.

Mr. Mabhouh played a role in the kidnapping and killing of two Israeli soldiers in 1989, and was involved in supplying Iranian weapons to Hamas.

Dubai is an open city, often used by intelligence officials for covert meetings. But Israel has also nurtured a quiet friendship with Dubai leaders.

Jim Krane, the author of “City of Gold,” a recent book about Dubai, said, “If Israel did authorize the hit, it either found Mabhouh’s elimination worth the damage to its relationship with Dubai, or the hit squad made a big mistake.”

Dubai officials suggested that the killers — whoever they were — did practice some sloppy tradecraft. Although the assassination was carried out without attracting notice, the suspects allowed themselves to be photographed repeatedly on surveillance cameras, sometimes ducking into bathrooms and emerging with fake beards but still recognizable, the Dubai police say.

Assuming that Israeli agents were responsible, Dubai may be the only place they could kill Mr. Mabhouh, said Robert Baer, a former C.I.A. operative in the Middle East. Assassinations took place there during the 1990s and drew little attention. But Dubai is now concerned about its reputation as a tourism and financial hub, and may have deliberately publicized the suspects’ identities — rather than handling the matter through private channels — to embarrass whoever planned the killing, he added.