By Jill McGivering
International human rights groups have called on the British government to prosecute a former Tamil Tiger militant commander who is detained in England.
Col Karuna used to be one of the Tigers' top commanders
Colonel Karuna Amman, who once ranked second in command in the Tamil Tigers, was arrested last Friday.
Human rights groups accuse Col Karuna of a range of crimes, including torture and the use of child soldiers.
They are gathering evidence against him to present to the British authorities in the hope that he will be prosecuted.
"Karuna has been implicated in serious violations of human rights and humanitarian law for quite some time," James Ross, the legal and policy director at the New York-based group, Human Rights Watch, told the BBC.
"He would be one of the top individuals in Sri Lanka who have been responsible for human rights abuses in that country. So he is someone we strongly believe should be prosecuted."
Mr Ross said several organisations were informally sharing information about the case, each working to gather evidence about different allegations against him.
Britain is a signatory to the international convention against torture.
So far there has only been one prosecution in the UK under the terms of this international convention, which led to the conviction of Afghan warlord Faryadi Zardad in 2005.
For a case to proceed, there has to be sufficient evidence that someone has been responsible for an act of torture themselves or can be connected to acts of torture.
The Tigers routinely deny recruiting underage fighters
That can be difficult to prove. Often there are no witnesses. The most likely evidence would be in the form of testimony from people who say they themselves have been victims of torture, carried out by Col Karuna or by people under his command.
So the challenge for human rights groups is to gather evidence which is strong enough to convince the British authorities that there may be grounds for prosecution and to present it to them while Col Karuna is still in the country.
There is also the issue of child soldiers.
Earlier this year, the New York-based Human Rights Watch published a report which accused Col Karuna's group of abducting and forcibly recruiting at least 200 Tamil children in Sri Lanka's eastern districts.
The government now says it is fully in control of these areas.
Human Rights Watch also accused the Sri Lankan government of failing to stop the abuse.
Unicef too has voiced concern in the past and called on the parties to stop this use of children in the conflict.
Reports suggest that most of the children recruited since 2006 are aged between 14 and 18. Some are younger than 12 years old.
Lucia Withers, the Asia programme manager for The Coalition to Stop the Use of Child Soldiers, told the BBC that the issue has been drawn to Col Karuna's attention repeatedly.
"He had command responsibility," she said, "but failed to act to stop it".
Until 2004, Colonel Karuna was the Tamil Tigers' top commander in the rebel-held east of the country.
In 2004, he split with the main part of the militant group and formed a breakaway faction.
He is later said to have collaborated with government forces and helped them to take control of territory in the east.
The nature of his relationship with the Sri Lankan government may prove critical.
Human rights groups say the Sri Lankan authorities have made no moves to prosecute him.
"Given that he's within UK jurisdiction and is unlikely to be prosecuted in Sri Lanka, we feel it's important for the UK government to investigate prosecuting him in the UK for violations of international law," Mr Ross said.