Key Points:

GAZA CITY - In the centre of Gaza City, a dense 2.5sq km of apartment buildings and orchards is barricaded by burned-out buses.

It is here, protected by gunmen, that Gaza's notorious Daghmash clan live, the prime suspects in the kidnapping of the BBC's Gaza correspondent, Alan Johnston, three weeks ago.

It is a neighbourhood at war with the Hamas-led Palestinian Government.

Somewhere in these streets, security sources believe, despite the Daghmash family's denials, the British journalist is being held.

His kidnapping, it is now thought, has its roots in the clan rivalries that are tearing Gaza apart - and in two murders that passed without notice outside Gaza, where such political killings have happened almost every week since Hamas won control of the Palestinian Government last year.

Ahmed Daghmash noticed. One of the dead, shot on December 19, was his brother Mahmoud. The other was a cousin, Ashraf. And so Ahmed, 35, an engineer, grabbed a gun and went to work extracting blood vengeance after Hamas refused to turn over his brother's killers.

This began a cycle of revenge that many believe has drawn in the BBC correspondent.

It is for this reason, say Hamas and Fatah officials hostile to the Daghmashes, that Johnston, 44, was grabbed - not as a political prisoner but as a bargaining chip to pressure the Government of Prime Minister Ismail Haniyeh to turn over the killers of Ashraf and Mahmoud to the Daghmash family for judgment under Islamic law.

And although there is no suggestion that Ahmed Daghmash has been involved in any of the kidnappings, his grievances are shared by his clan.

On December 19 Mahmoud and Ashraf Daghmash were armed and travelling with clan elders through the traffic-choked lanes of Gaza.

They were on a mission - to mediate a truce between Hamas and another local family, embroiled in Gaza's lethal feuds.

Mahmoud, perhaps, was not ideal for the mission. He was a bodyguard for an official with the Fatah movement. Hamas and Fatah had been in a state of conflict since the elections early last year. What is certain is that an angry encounter took place.

A dozen Hamas gunmen confronted Mahmoud, words were exchanged and the cousins were shot many times and left to die in the street.

The double murder sparked widespread fighting between Hamas and Fatah that in a few weeks would leave hundreds dead and wounded.

"If my brother had died in the fighting between Hamas and Fatah, then I would not take my revenge, for my brother was a soldier," Ahmed said in February. "But this was not a war. They just murdered him. Until there is justice, I will kill any Hamas official I can find."

They are frightening enough words from anyone. But Ahmed, a Russian-trained engineer, comes from a family that many in Gaza consider to be gangsters - controlling smuggling tunnels from Egypt into Gaza through which pass guns and other contraband.

It is a trade, the clan's enemies allege, that has made the family rich, well armed and politically independent. At first, the Daghmash family claim, they pursued the "legal route" and called on the Government to arrest the men responsible for the shootings.

But a top official in the Hamas-led "Executive Force", the law enforcement body of the Interior Ministry, explained that militant loyalties prevented such arrests.

"If you ask me once, I am the Interior Ministry police force," Abu Mutana said, explaining why no arrests were made. "If you ask me twice, I am the police of Gaza. But if you ask me a third time, my friend, I am Hamas, only Hamas."

Throughout February and March, Hamas officials were periodically killed or wounded in mysterious shootings, even after Fatah and Hamas had reached a ceasefire.

People said it was the revenge of the Daghmash, but when asked directly Ahmed was coy about the shootings.

But even as family elders would claim to have no direct knowledge of the killings, they would also argue that "if [Ahmed] has done this, then it is his right".

Ahmed is not the only clan member at war with Hamas. Far more sinister and dangerous are the family's ties to fundamentalist Islamic groups such as the Popular Resistance Committee and Jaish al-Islam (Army of Islam), both reportedly headed by Mumtaz Daghmash, whom Israelis think masterminded last summer's kidnapping of the Israeli soldier Gilad Shalit, still in captivity.

Mumtaz appears to have been the moving force behind a rash of kidnappings of Western journalists and aid workers in Gaza, in exchange for money and influence from the Palestinian Government.

Ahmed and a dozen Daghmash fighters are on patrol. Ahmed explains: "I will never forget the men who killed Mahmoud and never forgive. Fatah-Hamas? Their problems will never be solved because of this. They did not make the arrests, so I must kill the men who killed my brother." And Alan Johnston must remain a captive.

A career in the hot spots

Alan Johnston was born in Lindi, Tanzania, on May 17, 1962.

He was educated at Dollar Academy in Dollar, Scotland, and Dundee University and gained a diploma in Journalism Studies from the University of Wales in Cardiff.

Johnston joined the BBC in 1991 as a sub-editor in the World Service newsroom before becoming the BBC's correspondent in Tashkent from 1993 to 1995.

From 1997-98 he was Kabul correspondent.

Johnston became a programme editor on The World Today and then a general reporter in the World Service newsroom.

His three-year posting to Gaza as BBC correspondent began in April 2004.

He was due to leave Gaza when he was seized.