They turned into Friary Gardens, a crescent of small terraced houses, most of them shingled in grey, with front gardens that had been concreted over for the owners to park their cars and store their wheelie bins. The Carroll’s house was one of the few that was hidden behind a high privet hedge.

Katie and her detectives stood back while one of the uniformed gardaí went up to the front door and banged its brass knocker, hard. It was opened almost at once, but by less than ten centimetres, still on its security chain. Nolan Carroll, with his wave of white hair, peered out from the gloom of the hallway. He looked to Katie like a suspicious badger poking its nose out of its sett.

‘What do you want?’ he demanded. ‘It’s Sunday.’

Katie stepped forward. ‘Nolan Carroll, I’m here to arrest you on suspicion of the murder of Roisin Carroll. You are not obliged to say anything unless you wish to do so, but anything you say will be taken down in writing and may be given in evidence.’

There was a long silence. Nolan Carroll continued to look out at them, but made no move to slide off the security chain or to open the door any wider.

‘Would you please open the door, Mr Carroll, and step outside.’

‘It’s Sunday. It’s the Lord’s Day.’

‘I’m perfectly aware of that, Mr Carroll, but whatever day of the week it is, you’re under arrest. Now will you please open the door or else we’ll be obliged to open it ourselves by force.’

Nolan Carroll hesitated a moment longer, and then closed the door, which he would have needed to do in order to slide off the security chain. However, he didn’t open it again, and Katie heard him calling out to his wife. ‘Hilda! Hilda, where are you?’

‘Knock again,’ she told the garda.

The garda knocked six or seven times, and shouted out, ‘Open up, Mr Carroll!’

‘Is there any way he could sneak out round the back?’ asked Katie.

‘I’ll go down the side alley there and make sure that he doesn’t make a run for it out of his yard,’ said Detective Cullen.

Katie went up to the door, bent over and opened the letter-box flap. ‘Mr Carroll, you need to open the door and come out now! You’re only making things worse for yourself!’

There was still no response, so she stood back again and said to the gardaí, ‘Okay… kick it in, if you can.’

At that moment, though, the front door opened, and Mrs Carroll appeared, wearing a bulky red cardigan, her hair in nylon curlers.

‘What’s going on? What do you want here?’

‘Your husband, Mrs Carroll. He’s under arrest. Will you tell him to come out or else we’ll have to come inside and fetch him ourselves.’

‘You’ve no right.’