The son of a notorious terror chief is among those being questioned by police probing the murder of two soldiers in Northern Ireland.

Declan McGlinchey, 32, the son of Dominic "Mad Dog" McGlinchey, was held in Bellaghy, County Londonderry, bringing to nine the number of people being quizzed over the murders of the soldiers and a police officer.

Dominic once boasted of personally killing 30 people. He was a member of the IRA, but later joined the smaller INLA grouping and eventually led it. In 1994 he was shot dead in Drogheda, Co Louth.

In 1987, Declan saw his mother, Mary McGlinchey, killed when a gunman shot her nine times in her home.

She was also said to be an INLA member.

Police were attacked by gangs of masked youths after they arrested Republican Colin Duffy in Lurgan during the investigation into the murder of the soldiers.

Duffy, a 41-year-old former IRA prisoner, has broken away from mainstream Republicans and is part of a protest group critical of Sinn Fein's decision to back the Police Service of Northern Ireland.

He came to prominence in the 1990s after being acquitted of the murder of a soldier when it emerged a key witness against him was a Loyalist paramilitary. The witness, Lindsay Robb, was later murdered in the east end of Glasgow in a row over drugs.

Duffy was later arrested over the murder of two police officers but the case collapsed.

PSNI Chief Constable Sir Hugh Orde has claimed there are about 300 dissident Republicans involved in the campaign to destroy the Northern Ireland peace process. But he said the security forces had identified many of them and were working hard to put them behind bars.

Sir Hugh said: "In the past 18 months or so there have been at least 25 attempts by dissident terrorists to kill officers on and off duty.

"But we must put this into perspective. These groups are small in number.

"The Real and Continuity IRA are disrupted, infiltrated and disorganised.

"The current wisdom is that they number around 300 in a population of 1.75 million.

"When they try to gain any political foothold through elections, they are wiped out by decent politicians committed to moving Northern Ireland forward in keeping with the will of the people.

"They are also very dangerous, like any cornered animal in its death throes.

"We have identified many of them.

"We are working flat out with the security services and other specialists to disrupt and arrest them and lock them up for the rest of their natural lives.

"Their current focus, in my judgment, is to attack my officers and we will meet that head-on."

Sinn Fein's Martin McGuinness, the Northern Ireland Deputy First Minister, is in the US to meet President Barack Obama and business leaders.

Last night he denied stepping up his personal security over fears of attack.

He said: "I am not concerned about my safety and my security.

"I and others in the political process have the overwhelming support of the people of Ireland for what we are doing."

Detectives were granted a five-day extension yesterday to further interview a 21-year-old man arrested on Friday in connection with the investigation into PC Carroll's murder. Officers hunting the killers of PC Carroll have seized a gun and ammunition in Craigavon, Co Armagh.

A man was arrested in the Antrim area on Saturday over the murders of soldiers Mark Quinsey, 23, from Birmingham, and Patrick Azimkar, 21, from London.