A man in garda custody over the alleged plot to murder a Swedish cartoonist wrote an email to the mastermind of the conspiracy 'Jihad Jane' saying he had already twice attempted to become a martyr, saying: "I tried twice but I wasn't successful…[but] I will…try until Allah m[a]ke it easy for me."

A senior garda investigating the alleged murder conspiracy said there was a distinct possibility that some of those in custody could face extradition to the US to face charges.

Details of the email from the man based in Ireland are contained in the US court indictment documents of Colleen R LaRose – who also went by the pseudonym of 'Fatima LaRose' and 'Jihad Jane,' – which have been obtained by the Sunday ­Tribune.

LaRose has been charged with conspiracy to commit murder overseas, conspiracy to provide support to terrorists, making false statements and attempted identity theft.

Three men and a woman remained in custody yesterday in relation to the plot to murder Swedish cartoonist Lars Vilks.

The woman is an American citizen and the men are of Croatian, Algerian and Libyan nationality. Gardaí must return to the District Court this morning if they wish to extend their period of detention. A decision must then be made tomorrow whether to charge any of them or release them.

Two of the men are being held in Waterford city, the third in Tramore and the woman is in custody in Thomastown, Kilkenny. Three others have been released without charge and a file is being prepared for the DPP.

Detectives also recovered hundreds of pages of documents in Arabic when they raided seven houses, an office and a bakery in Waterford city, Tramore and Ballincollig in Cork city last Tuesday. This documentation is currently being translated.

Seized computers of some of the suspects in custody has established internet links with 'Jihad Jane' and detectives said the investigation was progressing well.

"There has been some information found of significant interest since the raids. It is now looking possible that some of those in custody could be extradited to the US. There is particularly strong legislation in the US in relation to international terrorism and they could face more serious charges over there," said a senior garda source.

"The American woman in custody has only been here for a few months. At the moment, we are trying to confirm that their identities are not false."

Specialists from the computer unit, attached to the Garda Fraud Bureau, were this weekend continuing to sift through downloaded files to determine the extent of the relationship between those in custody and 'Jihad Jane.' As well as emails, it is believed they corresponded regularly on Muslim websites.

Colleen R LaRose visited Ireland last September to recruit people for a plot to kill Vilks, who controversially depicted the prophet Mohammad with the body of a dog.

The cartoonist was put under police protection in 2007 after his drawings prompted al-Qaeda to put a $100,000 bounty on his head. The sketches, originally produced for exhibition, were later published by a Swedish newspaper.