Police, who arrested four people on Sunday in connection with the courthouse attack, described the bombing as “unbelievably reckless”.

The attack is believed to have been carried out by members of a group known as the Real IRA, or sometimes as the New IRA – the largest dissident republican group in Northern Ireland.

The Real IRA split from the Provisional IRA in 1997 after rejecting the latter group’s ceasefire that year. In 2012, it merged with other republican groups and is now sometimes called the New IRA. It calls itself simply “Irish Republican Army”.

A bomb warning was called into the Samaritans in the West Midlands on Saturday night and the area was being evacuated when the bomb exploded.

PSNI assistant chief constable Mark Hamilton said: “At around 7.55pm [on Saturday night], officers on patrol in Bishop Street spotted a suspicious vehicle and were making checks when, around five minutes later, information was received that a device had been left at the courthouse.

“We moved immediately to begin evacuating people from nearby buildings including hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club. The device detonated at 8.10pm.”

He added: “At this stage it appears as though the vehicle used had been hijacked from a delivery driver in the Quarry Street area a short time before the explosion.”

Two men in the 20s and two men aged 34 and 42 are currently under arrest.