On the evening of 18 February, a Palestinian engineer boarded a train in the eastern Ukrainian city of Kharkov.

Dirar Abu Sisi was the manager of Gaza's main electricity power plant. He hoped to obtain Ukrainian citizenship, and was travelling on the overnight sleeper to Kiev.

He spoke to a friend on the telephone just as the train was pulling out of the station. All was well, he said, he was settling into his bunk for the night.

But when the train arrived in Kiev the following morning, he was nowhere to be seen. Somewhere along the line, Dirar Abu Sisi had vanished.

'Agents, Secret Service'

"It was strange," said Andrei Makarenko, a young Ukrainian who shared a compartment with Mr Abu Sisi on the train.

Dirar Abu Sisi boarded a train at Kharkov station in Ukraine but failed to arrive at his final destination Shortly after leaving Kharkov, Mr Makarenko says, three men entered the compartment.

They wore plain clothes. One showed an ID badge claiming to be from the Ukrainian Security Service. They checked Mr Abu Sisi's documents and took him away.

"It was like in a film, like in a book: agents, secret service," Mr Makarenko says. It was the last he or anyone else saw or heard of Dirar Abu Sisi for more than a week.

Nine days later Mr Abu Sisi's Ukrainian wife, Veronika, received a phone call. It was Mr Abu Sisi. He was in prison, in Israel.

It took several weeks for the Israeli authorities to issue an indictment.

When they did, it accused Dirar Abu Sisi of hundreds of counts of attempted murder, and of being the brains behind a missile programme run by Hamas, the Islamist movement that controls Gaza.

The Israeli court has since released partial transcripts of Mr Abu Sisi's interrogation sessions, in which he apparently admits to helping Hamas increase the range and accuracy of their rockets, as well as helping the movement set up and run a military academy.

His lawyers, and his wife, say he has nothing to do with Hamas, and knows nothing about rocket technology.

They say Mr Abu Sisi's confessions were obtained under duress while in Israeli custody, and are meaningless.

The Israeli government has refused to comment on the case.

But according to the indictment Mr Abu Sisi acquired his skills as a rocket scientist while studying in Ukraine in the 1990s, in the city of Kharkov.

Mr Abu Sisi did indeed study in Kharkov during that period.

That is where he met his wife Veronika. But at the National Academy for the Municipal Economy, where he studied for his PhD, staff said the curriculum was exclusively concerned with civil engineering.

There were military educational facilities in Kharkov at that time, but the Ukrainian ministry of education says it can find no record of a Mr Abu Sisi having attended any of them.

Link to kidnapped solider

Indeed, in the Israeli transcripts of the interrogation, Mr Abu Sisi is quoted as saying he only visited the military academy on four occasions, for a couple of hours at a time: hardly enough for a rocket scientist.

Hamas spokesman Ghazi Hamad has not ruled out Dirar Abu Sisi being involved with the group In Gaza, Ghazi Hamad, a spokesman for Hamas, was cautious in his defence of the engineer.

"According to his family, he was not involved in any military actions," he said.

"He was just working in his field in electricity and normal things, but not with rockets or something like that."

But when pressed, he could not rule out the possibility that Mr Abu Sisi had been involved with Hamas' armed wing, without his family's knowledge.

Israel takes attacks on its territory extremely seriously. Hundreds of rockets have been launched from Gaza into Israel in recent years, causing numerous civilian casualties.

So why did the Israeli authorities think it was worth the risk of conducting an international abduction?

Yossi Melman, security correspondent for the Israeli newspaper, Ha'aretz, thinks they simply made a mistake.

"Dirar Abu Sisi is not that important," he said. "It's a mystery. I think he was chosen because he was an easy target. It's as simple as that.

"Sometimes the bureaucracy is looking for the lost coin, not where it was lost but where there is a light."

But there is another possibility.

In 2006, Hamas captured a young Israeli soldier in a cross border raid. More than five years later, Corporal Gilad Shalit is still presumed to be held prisoner at a secret location inside Gaza.

Israel has an almost sacred covenant with its service personnel: it does not abandon its soldiers. The Israeli government is desperate to get him back.

The Israeli authorities have never publicly made any connection between the Abu Sisi case and the ongoing efforts to secure the release of Gilad Shalit. But Mr Abu Sisi's lawyers say that the initial focus of his interrogation did concern the whereabouts of the kidnapped soldier.

Warning from Hamas

Is Dirar Abu Sisi simply the unfortunate victim of mistaken identity? Did he really have no contact whatsoever with Hamas, as his wife and friends claim?

Dirar Abu Sisi is currently being held in Ashkelon prison in Israel, a short distance north of Gaza That seems unlikely for an engineer in his senior position, given the fact that Hamas controls Gaza.

The transcripts of his interrogation point to the possibility that Mr Abu Sisi was getting more involved with Hamas than he wanted.

"I tried to cease my assistance to Hamas regarding improving missiles' range," he is quoted as saying.

According to the court transcripts, Mr Abu Sisi mentions one particular member of Hamas' armed wing on a number of occasions: a man by the name of Raed Sa'ad, who apparently tried to dissuade him from backing out of his work with Hamas and may have issued a veiled death threat.

"Raed Sa'ad's answer was that there are many fighters or many people who are killed in strange missions.

"When I asked him what he meant he said: 'Understand that any way you want to,' and added that a man who has children should be afraid for them."

A number of different sources have told the BBC - off the record - that Mr Abu Sisi was detained by Hamas shortly before he left Gaza for Ukraine.

These sources say he was told not to make the journey. Hamas has declined to comment on the matter.

The official Ukrainian response to Dirar Abu Sisi's disappearance has been one of ignorance.

Ukraine's Security Service, the SBU, has denied any role in, or prior knowledge, of his abduction whatsoever.

But Ukraine has made no official complaint against Israel through diplomatic channels. Security experts believe it is highly unlikely the Israelis could have abducted a man on Ukrainian soil without at least tacit permission.

It is not impossible that Dirar Abu Sisi was in fact a secret and senior Hamas operative.

However, the evidence uncovered in this investigation suggests that - whatever his dealings with Hamas may or may not have been - he was not nearly that important.