"It could be Mossad, or another party," police chief Dhahi Khalfan told AFP.
"Personally, I don't exclude any possibility. I don't exclude any party that has an interest in the assassination" of Mahmoud al-Mabhouh, Khalfan said.
"There were seven or more people holding passports from different European countries" in the group suspected of killing Mabhouh, he said.
He refused to name the countries, but added, "We are currently in contact with these European countries to verify the authenticity of the passports."
The hardline Hamas on Friday accused Israel of killing Mabhouh, who was found dead in his hotel room in Dubai on January 20, and vowed revenge.
Hamas has acknowledged that Mabhouh was in Dubai to buy weapons for Hamas in its struggle against Israel's occupation of Palestinian territories.
Mabhouh, born in Jabaliya in northern Gaza, was behind the capture of two Israeli soldiers, Avi Sasportas and Ilan Sa'adon, in separate operations in 1989. Both were later murdered.
Khalfan said that "it seems (Mabhouh) opened the door" of his room, letting his killers in. "Mabhouh was suffocated," he said, adding that "strangulation is possible."
According to Khalfan, Mabhouh entered the United Arab Emirates (UAE) a day before his death using a passport that did not bear his family name.
"We were not informed by Hamas about the visit," he said. "It is strange that a person of his importance travelled alone."
Khalfan met Palestinian consul general Hussein Abdul Khaliq in Dubai on Sunday to discuss the murder, saying police would "work day and night" to track down the suspects, the official WAM news agency reported.
Israeli newspapers, meanwhile, hailed the killing, with the rightwing English-language Jerusalem Post calling it "another blow to the 'axis of evil'" that will make it harder for Hamas to get arms into its Gaza stronghold.
On Sunday, The Times of London cited unidentified Middle Eastern sources as saying Mabhouh's body was found by staff at the luxury Al Bustan Rotana hotel.
It said Mabhouh was travelling on a false passport and on arrival in Dubai was followed by two men described by local police as "Europeans carrying European passports."
The hit squad injected Mabhouh with a drug that induced a heart attack, photographed all the documents in his briefcase, and left a "do not disturb" sign on the door, The Times said.
It added that the Hamas leader was on a mission to buy arms from Iran, and was tracked from the moment he boarded Emirates flight EK 912 from Damascus on January 18.
A founder of Hamas' military wing, Mabhouh was in charge of arms purchases for the group.