A key security operative of the Palestinian Islamist movement Hamas was under arrest in Syria tonight on suspicion of having helped an alleged Israeli hit squad identify Mahmoud al-Mabhouh before he was assassinated in Dubai, the Guardian has learned.

Palestinian sources in the Gulf confirmed Nahro Massoud, a Hamas security official, was in detention and under interrogation in Damascus in connection with the 19 January killing, which is now widely assumed to have been mounted by Israel's Mossad secret intelligence service.

Khaled Mishal, Hamas's political leader, denied the allegation. "It is not correct at all," he said, as Hamas sources suggested a disinformation campaign by the group's Fatah rival. But informed Palestinian sources insisted Massoud was being questioned amid mounting speculation that potentially senior Palestinian defectors may have assisted in the plot.

The authorities in Dubai have refused to name two other Palestinians who were extradited from Jordan to the United Arab Emirates after the murder. Al-Jazeera reported that one of them worked for Fatah's security services but the Palestinian Authority has denied that.

Today fresh details of the assassination operation and the motives for the killing continued to emerge.

The plot

After returning from a walk around the city al-Mabhouh was ambushed in his room on the second floor of Dubai's al-Bustan Rotana hotel by four men wearing baseball caps and T-shirts identified by Dubai police as the "execution team". They were said to have broken into his room, suffocated him and departed in around 10 minutes. They then left the country.

The squad had constantly changed their appearances, using wigs, hats and glasses as disguises. But today questions emerged over the CCTV evidence released by Dubai police.

All the assassins began leaving the hotel around 20 minutes after the ­murder, according to Dubai authorities, who showed footage of Gail Folliard, claimed to be the only woman in the group, exiting the hotel. Police also released footage of what now appears to be a second woman leaving the building around the same time. She was wearing white trousers and a stripy hat, departing with a different man.

An Emirati official told the New York Times the hit team was not restricted to the 11 individuals whose photographs were initially released by Dubai police, but in fact included a total of 17 people, six of whom had not yet been identified. Those additional suspects could include the two Palestinians in UAE custody, the second woman shown in the footage and, potentially, Massoud.

The Palestinian connection

Massoud's name first surfaced in the Kuwaiti newspaper al-Siyasah which quoted "a well-informed source" in Dubai as saying he had been with Mabhouh until he was killed, raising questions about his involvement.

"Hamas suspects that he [Massoud] passed on the information that led to ­Mabhouh's killing," a senior ­Palestinian source told Israel's Ma'ariv newspaper. Massoud, a commander of Hamas's armed wing, the Izzedine al-Qassam brigades, fled from Gaza to Egypt where he was detained but released in late 2007, to the fury of the western-backed Palestinian Authority, run by Hamas's rival, Fatah.

Killings of Palestinians by Israel have often involved Palestinian agents being used to identity the target.

Israeli media have reported previously that Mabhouh's killing prompted Hamas to launch an internal investigation to determine whether Israel had infiltrated its ranks. "The assassination of someone as senior as Mabhouh has rung an alarm bell in Hamas," one official told the Jerusalem Post. "Only a few people in the Hamas leadership knew about Mabhouh's secret activities and movements."

But Yossi Melman, the Haaretz newspaper's expert on intelligence, commented that such stories were "another example of the sort of psychological warfare against Hamas that would have the organisation become even more suspicious of flawed security in its ranks".

Hamas has said little about Mabhouh except to acknowledge he was a valued member of the organisation. It immediately vowed to retaliate for his death.

A colleague told a Gulf newspaper: "Mabhouh played a key role in supplying the Palestinian people with weapons and money. His central role in the 2008-2009 Gaza war was clear, he supplied Palestinian fighters with special weapons, he was an important figure for our military."

Hamas has accused Israel of trying and failing to kill Mabhouh three months ago. Speculation about the motives for his assassination has focused on his role in the 1989 abduction and killing of two Israeli soldiers.

The Iranian connection

Middle East experts and diplomats see the Dubai plot as part of a wider clandestine struggle between Israel and Hamas – and a deliberate attempt to weaken the Palestinian organisation's links with Iran. Israel considered Mabhouh to be the point man in smuggling longer-range Iranian rockets into Gaza that would be capable of striking Israel's urban heartland.

Neither Hamas nor Dubai police have given any indication of why Mabhouh was in the emirate, a convenient access point for the Islamic Republic. Hamas has occasionally confirmed that its men have been sent to Iran for training by the Revolutionary Guards, confirming allegations made by Israel.

Iran says it supports Hamas as a resistance movement. But it is shy of revealing any information about its financial and logistical backing for the Palestinians. A rare public glimpse of this largely secret war came last January when Israeli planes and drones attacked a convoy of arms and explosives heading for Gaza across the Sudanese desert during the three-week Cast Lead offensive.

The Austrian connection

There were more signs tonight the plot was connected with a less obvious country, after claims Austria was used as a communications hub by the squad.

Dubai police identified Austria as ­"command centre" for the assassins, after mobile phone data showed at least seven numbers originating there. The gang never telephoned each other directly other than with what Dubai police speculated were "encrypted" messages, but contact was maintained via several Austrian mobile phone Sim cards. Austria has confirmed its officials were assessing the claims.

Vienna has a long reputation as a centre of espionage, having been used extensively during the cold war and elements of the sophisticated infrastructure have remained in place. In December 2008 radical Islamic terrorists also coordinated their bomb attacks in Mumbai, in which 160 people were killed, using Austrian mobile phone numbers.