The man arrested over the arson attack that killed a couple and two of their children in Ireland had recently been released from prison and is believed to have worked for a Traveller gang.
Kim English, her husband Philip, and their sons Callum, 12, and Jayden, nine, died in a petrol bomb attack at their home early on Tuesday morning in the Birchwood estate, just under two miles from Waterford city centre.
Philip and Kim's two other children, Nadia, 14, and Elliot, six, remain in critical condition in hospital.
The chief suspect was questioned by gardaí last night after he and a female alleged accomplice were arrested.
The man, aged in his 20s, was released from prison in recent months after being locked up for burglary, according to reports.
Gardaí suspect he carried out the attack after being contracted by a Traveller gang.
'This man is very well-known to gardaí in the area,' one senior source explained.
'He's a hired hand for several gangs in the area and has a history of carrying out attacks on people who owe drug debts in the city.
Philip and Kim English with their children, including Callum, left, and Jayden, centre
Kim and Philip (pictured) and two of their sons died in the early hours of Tuesday morning in the Birchwood estate, 3km from Waterford city centre
Forensic officers at the scene of an overnight fire in the Birchwood estate in Waterford City
'What you have to remember is that there's a lot of different drug dealers operating across the city. It's not like you belong to one gang. It's all very fluid.
'What gardaí believe is that one gang, which is headed by three brothers, hired this man to carry out the petrol-bomb attack. Gardaí have also secured CCTV of the vehicle they believe was used in the attack. An alert was sent around Garda stations to be on the watch for it.
'I believe he was in a highly distressed state when he was arrested.'
His alleged accomplice, a woman in her 30s, was detained by detectives on suspicion of acting as the driver in the attack.
The two are being detained under anti-gangland legislation, and they can be held for up to seven days before they must be charged or released.
The arrests followed several raids on properties linked to gangland criminals in the area.
Gardaí seized phones, laptops and vehicles on Wednesday. They will be forensically searched for any evidence relating to the crime.
It was previously reported that the petrol bomb had been thrown into the English's home, but detectives now believe the family's electric car was struck, causing it to explode into the home and causing a devastating fire.
Philip's son from a previous marriage, John James English, revealed that gangland figures in the city had terrorised his family for years over a £17,000 drug debt owed by someone known to the family.
Only three weeks before the fatal attack, the family car had been targeted with a petrol bomb. Nobody was injured in that incident.
John James also revealed that the home of his mother Anne, Philip's ex-wife, was previously targeted in another petrol-bomb attack.
On Wednesday afternoon, James English, an uncle of Philip, said: 'Well, whoever knows, you'd want to turn up, and own up to whoever's done it.
'At the end of the day, we have a lot of the family now wiped out.
Police at the scene of an overnight fire in the Birchwood estate in Waterford City, Ireland
A burnt-out window in the family home in the Birch Avenue
Floral tributes outside the English family's home
People gather ahead of a balloon release ceremony in honour of the English family
'Please, whoever's done it, own up. Be a man and own up for what you have done.'
A Garda spokesman yesterday confirmed the arrests.
He said: 'An Garda Síochána continue to investigate the multiple murder following the arson attack and fatal house fire that occurred in the Birchwood area of Waterford city shortly after 5am on Tuesday, October 13.
'This afternoon, Thursday October 15, investigating gardaí have arrested a male (aged 20s) and a female (aged 30s) for alleged offences contrary to section 72 Criminal Justice Act 2006 in connection with the arson attack and murder of four persons at 7 Birch Avenue, Birchwood, Waterford.
'Both individuals are currently detained under section 50 Criminal Justice Act 2007 in a Garda station in the Garda eastern region.
'The two children admitted to Children's Health Ireland Crumlin continue to receive treatment for their injuries. Their condition remains critical.
'The house at Birch Avenue remains a crime scene and subject to a technical examination by the Garda Technical Bureau.
'Postmortems on the four deceased have been completed. The postmortem results are not being released for operational reasons. Family Liaison Officers continue to engage with the bereaved family, who have been updated.
'Garda Síochána continue to appeal to any person, if you have any information on the murderous arson attack, please speak to the Garda investigation team at Waterford Garda Station, or contact the Garda Confidential Line at 1800 666 111, or speak with any member of An Garda Síochána that you know.
'Gardaí continue to appeal to any person who was driving in the vicinity of Birchwood, or the adjoining roads, between 4.30am and 5.30am on Tuesday, who may have dashcam footage or images, to make such footage available to investigating gardaí.'