Last week investigators exhumed the remains of the mother of two after a tip-off that a suspect may have placed a letter under the pillow in her coffin. The note, one of several documents retrieved, has been removed for technical analysis by forensic experts.

Gardai, who did not re-examine O’Reilly’s remains, initially feared that the letter would be illegible because it was so damp. However, garda technical experts have chemically enhanced it and are hopeful that they will be able to read it with the assistance of infrared technology.

“It is not uncommon for family or friends to place letters and other items in a loved one’s coffin before burial, ”said one senior garda officer, who said the letter could also identify a motive for her savage murder. “We are hopeful that this particular letter will significantly enhance this investigation.

“At first, the witness who tipped us off didn’t attach any significance to the letter, but we believe it could be crucial and will assess its impact later this week.”

Requests for a murder victim’s body to be exhumed are examined carefully and only granted when there is compelling evidence it could lead to a breakthrough.

Most of the work undertaken by forensic document analysts revolves around the identification of handwriting.

Gardai will initially analyze the type of ink and paper used by the writer. The second stage, which detectives believe could solve O’Reilly’s murder, will involve the visual examination of handwriting in order to establish the identity of the writer.

Rachel, 30, was battered to death with a blunt instrument at her home in Baldarragh, near Naul, Co Dublin, on October 4 last year. O’Reilly’s body was exhumed from her grave at Balgriffin cemetery at 6.30am last Tuesday. The coffin was resealed two hours later and her remains reinterred without being subjected to DNA tests.

Last week Marie Cassidy, the state pathologist, told the Tallaght coroner’s court that the cause of O’Reilly’s death was blunt trauma to the head, inhalation of blood, a fractured skull and injury to the brain. The inquest into her death has been adjourned until September 15 at the request of Inspector Sean Galway from Balbriggan garda station.

Three people, including Joe O’Reilly, Rachel’s husband and a self-confessed suspect in her murder, have been questioned in connection with her death. O’Reilly and two colleagues, Declan Quearney, a former soldier, and Nicola Pelley, a sales executive, were all released without charge last November.

Investigators have taken more than 700 witness statements in one of the largest murder investigations in recent times, but most of the evidence has been circumstantial.

CCTV footage has been examined and sent to overseas technical laboratories for further analysis.

Exhumation orders are rare in Ireland, and typically ordered if it is suspected that a victim may have been poisoned.

There have only been three such orders since the body of John Gethings, an elderly patient at Naas General hospital, was exhumed in July 2003.

Gethings was suspected of dying of an overdose administered by a nurse at the hospital. A subsequent toxicology report on the exhumed corpse found doses of Serenace, a sedative, in the body of the 77-year-old grandfather.