Israel admitted this week that it was behind the abduction of a Gazan engineer who went missing more than a month ago while travelling on a train in the Ukraine.

Israeli officials confirmed that Dirar Abu Sisi, 42, was being held in Israel's Shikma prison, near Ashkelon, after a judge partially lifted reporting restrictions late on Sunday. However, the reasons for Abu Sisi's abduction are still covered by the gag order.

The whereabouts of Abu Sisi, the operations manager of Gaza's only power plant, have been the subject of intense speculation since he disappeared on February 18 as he travelled on a train to the Ukrainian capital, Kiev.

Suspicions that he might have been spirited to Israel were raised by the Ukrainian spokesman of the United Nations High Commissioner for Refugees earlier this month.

A few days after his disappearance, Abu Sisi's wife, Veronika, a Ukrainian national, accused the Israeli spy agency Mossad of kidnapping him to extract information that could be used to disable Gaza's power station in a future confrontation with the enclave's Hamas rulers.

Israel bombed the plant during its three-week military assault, Operation Cast Lead, in winter 2008, causing blackouts across much of Gaza. Israel also targeted the power station in June 2006, cutting power to 700,000 Gazans for several months while it was fixed at a cost of more than $5 million.

Abu Sisi's family suggested another reason why Israeli might consider him a high-value target. They say he had recently developed a method to reduce the plant's dependency on high-grade diesel fuel, the flow of which Israel controls into Gaza.

In January Hamas officials announced that the station's turbines had been modified to work on regular diesel, which is cheaper and can be smuggled in through tunnels from Egypt.

The Israeli media, on the other hand, have speculated that Abu Sisi must be a senior Hamas activist to have secured an important post at the plant. The family have denied the claim, saying he was not involved in any political faction and was appointed because of his skills as an engineer.

One of his Israeli lawyers, Smadar Ben Nathan, who met him for the first time at the court hearing on Sunday to lift the gag order, said she believed Israel had carried out the operation based on false information.

She called the abduction a "miscalculation", saying interrogators had dropped their original line of questioning. She said the gag order meant she could not discuss the case further.

No charges have been brought yet. Ben Nathan said her client had lost a great deal of weight and his health was deteriorating after more than a month incommunicado. His family is concerned that he is being tortured.

Although the Mossad is suspected of carrying out many assassinations on foreign soil -- including a hit on a Hamas leader, Mahmoud al Mabhouh, in a Dubai hotel last year -- there are few examples of it seizing individuals in foreign countries to bring them to trial.

Ben Nathan said she could identify only two similar cases: Israeli agents captured the Nazi war criminal Adolf Eichmann in Argentina in 1960, and smuggled Mordechai Vanunu, a nuclear whisteblower, out of Italy in 1986.

Victor Kattan, an international law expert at the School of Oriental and African Studies at London University, said Israel had broken several human rights laws in seizing him rather than invoking treaty agreements between the Ukraine and Israel and requesting his extradition.

The Palestinian Centre for Human Rights, based in Gaza, said it had learnt the details of how Abu Sisi was seized from another Israeli lawyer who had access to him. They did not name him, apparently because the information was a violation of the gag order.

In a statement, the centre said three men, two of them apparently wearing official Ukrainian uniforms, had dragged Abu Sisi, hooded and handcuffed, from his carriage at a stop en route to Kiev, where he was due to meet his brother.

He was later interrogated in an apartment by six people who identifed themselves as Mossad agents, before being put on a plane. The flight took about four hours. The plane then made another flight of about an hour to Israel, the PCHR said.

Abu Sisi's brother, Yousef, accused Ukraine of being "deeply involved", adding that he had spent three weeks being "kicked like a football from one office to another" as he sought help from the police and various intelligence agencies. "At one point an official even threatened to make me disappear," he said.

Abu Sisi was in Ukraine to apply for citizenship so that he and his wife could immigrate with their six children, his brother said. "He was desperate to leave Gaza and and take his children to Ukraine away from the Israeli bombs and attacks. How could he be a threat?"

According to Veronika, her husband had encountered problems at an interior ministry office in the city of Kharkiv earlier on the day of his disappearance. Officials there had briefly refused to return his passport.

So far Ukraine has kept a low profile on the incident.

During an official visit to Israel last week by the Ukrainian prime minister, Mykola Azarov, reporters due to cover his meeting with Benjamin Netanyahu, Israel's prime minister, had their invitations withdrawn at short notice and without explanation.

However, during an interview with Israel's Haaretz newspaper while he was in Israel, Azarov responded to a question about Abu Sisi's disappearance: "We don't have clear information right now … I don't want to imagine that such things are carried out on the soil of a friendly state."

Abu Sisi's kidnapping was first brought to light by Richard Silverstein, an American blogger.

The Association of Civil Rights in Israel, which sought the removal of the gag order, said in its petition to the court: "It is inconceivable that the authorities in a democratic country be able to secretly arrest people and ‘vanish' them from the public eye."

Jonathan Cook is a writer and journalist based in Nazareth, Israel. His latest books are "Israel and the Clash of Civilisations: Iraq, Iran and the Plan to Remake the Middle East" (Pluto Press) and "Disappearing Palestine: Israel's Experiments in Human Despair" (Zed Books). His website is www.jkcook.net.