Police officials say tests showed a substantial amount of a fast-acting muscle relaxant called succinylcholine was found in Mahmoud al-Mabhouh's bloodstream.
The analysis was part of tests conducted after al-Mabhouh was killed Jan. 19. Dubai authorities have blamed a hit squad organized by Israel's Mossad secret service.
The drug is sometimes used to administer a breathing tube or anesthesia.
Dubai police on Sunday also said a third Palestinian suspect is in custody. At least 26 other suspects are wanted after being accused of traveling on fake European and Australian passports.
Authorities there have either declined to say whether they are investigating, or told The Associated Press they have no reason to hunt down the 26 suspects implicated in the Jan. 19 killing of Mahmoud al-Mabhouh.
European countries' reluctance to investigate may have something to do with the widely held belief that the killing of al-Mabhouh was carried out by a friendly country's intelligence agency — Israel's Mossad. The Jewish state has previously identified him as the point man for smuggling weapons to the Gaza Strip's Hamas rulers.
Experts say arresting Israeli agents — or even digging up further evidence that Israel was involved — could be politically costly.
"I would guess that it's in the political interest of certain countries not to get proactive in this case," said Victor Mauer, deputy director of the Center for Security Studies at Zurich's Federal Institute of Technology.
"Countries such as Germany have a special relationship with Israel because of their history and therefore wouldn't be interested in investigating," he said.
Switzerland's federal prosecutor's office says it has "no evidence relating to this case that would justify opening an investigation," although Dubai police insist that eight of the suspects fled Dubai for Zurich.
Dutch and Italian officials, too, said they aren't investigating the flight of six suspects to Amsterdam and Rome.
All three countries say they haven't received an official request for help from Dubai yet, though authorities there have asked Interpol to circulate arrest warrants that the Arab emirate issued for 11 suspects charged with "coordinating and committing the murder."
Dubai police did not respond to repeated requests by The Associated Press for comment on cooperation with European and other police agencies in the investigation. But Dubai Police Chief Lt. Gen. Dahi Khalfan Tamim was quoted by the Dubai-based Al Bayan newspaper Saturday as saying that an international security team has been created through diplomatic channels to aid in the cross-border hunt. No other details were given in the report.
France, meanwhile, has said it is only probing the alleged use of three French passports in the crime. Two suspects landed in Paris on Jan. 20. Their trail ends there.
Germany, too, is investigating the possibility that a forged or illegally obtained German passport was used. But prosecutors in the German city of Frankfurt, where four suspects returned, say they aren't investigating the killing itself and don't see any reason to, as most likely no crime was committed in Germany.
Philip Alston, an independent U.N. human rights investigator and New York University law professor, said European countries would be wrong to ignore the case.
"If a foreign intelligence agency was responsible for the killing of al-Mabhouh, the matter should clearly be classified as an extrajudicial execution," he said. "All states have an unquestioned obligation to investigate and prosecute anyone accused of a killing who they have reason to believe is within their jurisdiction. Political considerations can never be invoked to avoid taking the necessary action."
Michael Boyle, a lecturer in strategic studies at the University of St. Andrews, Scotland, said European countries were "unlikely or unwilling to make it a serious political issue with Israel. It's going to raise up embarrassing questions and complicate their relationship with Israel," he said.
Boyle said the focus on forged passports made sense, however. "I think there's a concern on the part of European governments that if this process of Israeli operatives using European passports were to continue, that would put European citizens at risk."
Britain has sent a special police investigator to Israel to meet with six dual nationals whose passports were used in the assassination, even though they weren't in Dubai at the time. Australia and Ireland also have sought clarifications from Israel on the alleged use of their passports.
Only Austria has gone further and investigated whether Austrian SIM cards were used. Interior Ministry spokesman Rudolf Gollia declined to provide details about the findings except to say there are no indications that there was a "command center" in Austria, as Dubai police have claimed.
Boyle said there was "a certain degree of moral revulsion and disgust" among European countries "because this is a murder."
But "there's an understanding that these sort of intelligence operations get conducted, but don't get caught with it and don't make it politically costly for us. I think that's the real story of the Dubai incident."