The identity of the Manchester suicide bomber who killed 22 people and injured 59, was quickly established after police found one key piece of evidence in his pocket.
The young man, named by British police as Salman Abedi, exploded a homemade bomb and died inside Manchester Arena at the end of Ariana Grande's performance on Monday night (local time).
UK police revealed Abedi was a 23-year-old British national of Libyan descent. He was born in Manchester and grew up alongside three siblings.
A US intelligence official told NBC that Abedi's bomb was "big and sophisticated" and used materials hard to obtain in Britain, which pointed to help from others.
Abedi also had ties to al-Qaeda, according to the official, and received terrorist training abroad.
Authorities were able to identify him using a bank card found in his pocket and confirmed his identity using facial recognition technology.
The official said members of Abedi's own family had informed on him in the past, telling British authorities he was dangerous.
According to ABC News in the US, British counterterrorism authorities had already identified Abedi as a potential threat, along with hundreds of others.
"Abedi was a terrorist suspect in the UK, MI5 were aware of him," Robin Simcox, a terrorism and national security analyst at The Heritage Foundation said.
"They were aware that he posed a potential threat but they didn't think he posed an imminent threat that he proved himself to do in Manchester."
Intelligence agents are now investigating reports the football-obsessed Abedi slipped into Syria while visiting relatives in Libya several times in recent years, The Sun reports.
His home in the Manchester suburb of Fallowfield was one of two that police raided in relation to the attack.
To outsiders Abedi was seen as the quiet one in the family. According to the Daily Beast, his older brother Ismael was the more outspoken one of the boys and his sister Jomana was known for her glamorous social media accounts that channel Keeping Up With The Kardashians.
His father, known as Abu Ismail, was a prominent member of the community, who used to perform the call to prayer at Didsbury Mosque and Islamic Centre.
The imam of the mosque, Mohammed Saeed told ABC, that Abedi became angry with him after he gave a sermon in 2015 in which he criticised Islamic State.
"He was showing me hate, he hated me basically," Saeed said. "I was shocked, shocked and angry. All innocent lives matter."
The mosque is located in an area of Manchester known as Moss Side, which one expert told ABC was considered to be a hot bed of Islamic State recruitment.
Neighbours say Abedi, who studied business and management at Salford University before dropping out, had grown a beard in the last 12 months and had begun acting strangely.
"A couple of months ago he [Salman] was chanting the first kalma [Islamic prayer] really loudly in the street. He was chanting in Arabic," Lina Ahmed told The Sun.
"He was saying 'There is only one God and the prophet Mohammed is his messenger'."
Another neighbour, Leon Hall, said the young man "had an attitude problem."