MANCHESTER, UNITED KINGDOM — British officials are probing how the suicide bomber behind Monday night’s attack in Manchester was able to prepare for and execute the atrocity.

The attack — which killed 22 people and injured 59 — took place during a concert by American singer Ariana Grande at Manchester Arena on Monday evening. According to the Telegraph, the suicide bomber, identified as 22 year-old Salman Abedi, detonated what appeared to be a nail bomb in the arena’s foyer.

British security services are investigating how Abedi came into possession of the bomb.

According to the BBC, one possibility is that Abedi was taught how to make the explosive by someone linked to ISIS or al-Qaeda. He reportedly traveled to Libya as recently as three weeks ago, and may have learned there.

Another scenario is that he was self-taught and may have built the explosive himself. However this would have required knowledge of engineering.

Abedi may have also been given the device, which suggests a skilled bombmaker may be resident in the UK.

British Prime Minister Theresa May this week raised the threat level country’s terrorism threat level to “critical,” meaning another attack could occur at any time.

The last terrorist bombing in UK happened in 2005, with recent attacks involving the use of knives or vehicles.