Friends of Derek Glennon, who was killed in jail last Monday, threw a deadly Yugoslav-made fragmentation grenade at the home of an innocent associate of the man charged with his murder. The grenade, designed to kill anyone within 20 feet, exploded but luckily there was nobody in the house at Slane Road last Thursday night.

The device left one crater the size of a football at the spot where it exploded and propelled over 25 massive pieces of shrapnel into the house.

Detectives fear that this incident could be the first of many and have identified six houses that could be targets for Glennon's friends.

The night before the grenade attack, a number of houses around the Crumlin area were targeted by people looking to find a close relation of Declan O'Reilly. O'Reilly (27) has been charged with Glennon's murder.

Garda� are on a state of high alert this weekend to prevent any more trouble but fear that a new feud has now begun.

A senior source said: "The ante has been upped by the type of grenade that was used. This fragmentation device kills anything it comes into contact with and would literally tear a body apart. It was probably smuggled in by one of the Limerick gangs and it shows that Glennon's associates mean business and are determined to avenge his death."

Glennon (24) was stabbed to death after an argument on the D-wing landing of the jail last Monday evening. He had been jailed for five years in January 2004 after admitting to the unlawful killing of a cyclist at Davitt Road on 16 December 2002. The criminal had been travelling at speed in a stolen car when he struck and killed Neil King (26).

In October 2005 three prison officers escorted Glennon to St James's Hospital for treatment. He was smoking outside the main building when a man appeared armed with a gun and pointed a weapon at the guards.

He threatened to "blow the head off" a prison officer if he did not uncuff the prisoner. Both Glennon and his associate escaped in a waiting car.

Originally from Crumlin, Glennon was seen associating around the Cabra area after he escaped. He got into a dispute with a local drug dealer in a fast-food shop and vowed to have the man killed. On 12 December 2005 Glennon walked into the Fassaugh House pub in Cabra at around 1am and shot a 23-year-old man in the head, leaving him critically injured in hospital. The man was entirely innocent and was the victim of mistaken identity. Glennon calmly walked out of the pub believing that the man was dead.

The victim managed to recover from his injuries and was in hospital when he saw a newspaper report about the missing Derek Glennon. He recognised Glennon's picture in the newspaper as the man who had shot him and contacted garda�.

Glennon was eventually arrested by Mountjoy and Kevin Street detectives for stealing a bicycle and was questioned about the shooting. Although he denied it, he was positively identified by his victim. However, the DPP ruled that there was insufficient evidence to charge Glennon and the incident officially remains unsolved.

Garda� and prison staff regarded Derek Glennon as a particularly nasty individual. He had been disciplined on over 50 occasions for breaches of discipline while in Mountjoy and he was known to be friendly with members of the Dundon crime family from Limerick.

Garda� suspect it was the Dundon/ McCarthy/Ryan gang which supplied the fragmentation grenade used in Thursday's attack. There were four incidents involving explosive devices in the capital last week.

Derek Glennon's death in Mountjoy last week has provoked political outrage and is just the latest incident to have rocked the prison service in recent months. Two weeks ago, a convicted armed robber was mistakenly released after a man of the same name was due to be set free. He has still not been found.

Last month, 42-year-old Juan Carlos Alba escaped from a work party in Ballymun. He is nicknamed "the Chemist" and was sentenced to eight years in prison in July 2004 for his role in an international cocaine-trafficking operation. It is believed that he is now out of this jurisdiction.

Listeners to Joe Duffy's Liveline programme on RT� were amazed to hear a dangerous criminal from Finglas, John Daly, phone in to the show from his cell in Portlaoise last May. A subsequent investigation found that some of the country's most dangerous prisoners had flatscreen TVs in their cells and kept budgies as pets.

The prison service has launched investigations into all these security breaches.