Two Iraqi men are to stand trial in Baghdad charged with involvement in the kidnap and murder of Margaret Hassan, the British aid worker killed in Iraq four years ago, The Times has learnt.

The net is also closing on a third man suspected of shooting Mrs Hassan, 59, in the back of the head as she stood blindfolded, gagged and helpless. A fourth man, Mustafa Salman al-Jubouri, was given a life sentence in June 2006 for his part in the crime. It was reduced to 18 months on appeal, according to the family of Mrs Hassan, who had British, Irish and Iraqi nationality.

The alleged ringleader, however, remains on the loose. He is believed to be in hiding in Syria. A number of other suspects are also at large.

An Iraqi police captain, who has been working on the case since Mrs Hassan was snatched from her car by eight gunmen in Baghdad on October 19, 2004, is determined to bring everyone to justice. “I gave my word,” he said. “Even if it means spending every last moment of my career working on this, the case is never closed. It will remain open until we get everyone.”

Adding to the challenge, the whereabouts of Mrs Hassan’s body remain a mystery. Police dug up a site, south of Baghdad, last month after a tip-off but it turned out to be a hoax. “We are closer now than at any time to finding her so that she can be brought back to her family,” the captain said.

The trail of the kidnappers went cold for much of 2006 and 2007 but, in the first in a series of breakthroughs this year, British Special Forces and Iraqi forces arrested one suspect in a joint raid seven months ago.

The man, 26, an English-speaking engineer, has admitted being the interpreter for the al-Qaeda-linked gang that seized Mrs Hassan, according to the police captain. “Her Arabic was not good enough so they used him to talk to her,” he said. Mrs Hassan was married to an Iraqi man and had lived in Baghdad for 30 years.

She was working for the aid agency Care International at the time of her capture.

The kidnappers wanted to know if she was Jewish and if she was spying for the British. “They thought she was passing intelligence to coalition forces, which is not true. Margaret Hassan was helping hospitals,” said Major-General Abdul-Karim Khalaf, a spokesman for the Interior Ministry.

A second man, 25, was detained in an Iraqi operation in Baghdad three months ago. He is also suspected of being part of the group involved in the kidnapping. “The trial will be very soon,” the captain said. Revealing new details of the abduction, he said a failed attempt had been made to kidnap Mrs Hassan five days earlier.

In the successful swoop two cars containing eight gunmen ambushed the aid worker as she was travelling to work in Baghdad. Her driver was badly beaten and an Iraqi colleague, sitting with her, had a gun pointed at his face. Mrs Hassan was pulled from her car, shoved into one of the kidnappers’ vehicles and driven away.

She was held nearby for two days before being transferred to a farmhouse in Arab Jabour, a hostile stretch of palm groves and fields south of Baghdad. It was in this area that she was murdered 15 days later. He said he has worked closely on the case with British police officers based at the British Embassy in Baghdad.

One British officer went with the captain and some Iraqi forces in early 2005 to the farmhouse where Mrs Hassan had been held. The murder gang is thought to have been behind the kidnappings of 14 foreigners, including two French reporters, three Romanian journalists, an Italian and a Spanish reporter.

Captivity and death

October 19, 2004 Margaret Hassan kidnapped in Baghdad. Anonymous captors release video

October 20 Husband, Tahseen Ali Hassan, appeals for her release

October 22 Second video released. Hassan appeals to Tony Blair

October 25 Hundreds protest in Baghdad for Hassan’s release

October 27 Third video shows Hassan confused and close to tears

November 2 Al-Jazeera declines to show a fourth video in full on humanitarian grounds

November 16 Final video shows a blindfolded Hassan being shot

March 2005 Three men arrested in connection with the kidnapping

January 31, 2006 A leaked Italian police report suggests captors wanted $10 million (£6.5 million) ransom

June 5 Mustafa Salman al-Jubouri is jailed for life for his role in the murder. Sentence later reduced to 18 months

Source: Times archives

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