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Police officers stand near the Manchester Arena in Manchester, United Kingdom, May 23, 2017. Reuters

Hashem Abedi, the younger brother of the man who bombed the Manchester Arena Monday, was arrested Wednesday on suspicion of having links to the Islamic State, Reuters reported. Abedi was apprehended by police in Tripoli.

Salman Abedi, 22, has been identified as the suicide bomber who died at the scene of the Monday attack that killed 22 and injured more than 60. ISIS quickly stepped forward to claim responsibility for the bombings, though police said they had not yet verified the claims.

Another brother, Ismail Abedi, 23, was arrested by counter-terror officers in the U.K. Tuesday.

Read: UK Raises Terror Threat To Highest Possible Level: What A Critical Alert Means

The son of Libyan refugees, Salman Abedi was previously known to security in the United Kingdom. The bomber reportedly made trips to Libya in the past, including one just before the attack, where he became connected with ISIS. A United States security official cited by NBC News said his family had warned authorities about him in the past, saying he was “dangerous.”

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Police officers stand near the Manchester Arena in Manchester, United Kingdom, May 23, 2017. Reuters

The attacker’s father, Ramadan Abedi, came forward to discuss the incident.

“We don’t believe in killing innocents,” he told the Associated Press from his home in Tripoli. “This is not us.” He also confirmed to reporters that police arrested another one of his sons.

Some neighbors stepped up and described the family as having acted “strangely” in the days before the attack.

“They are a Libyan family and they have been acting strangely,” said a neighbor, Lina Ahmed, according to the Telegraph. “A couple of months ago [Salman] was chanting the first kalma [Islamic prayer] really loudly in the street. He was chanting in Arabic. He was saying ‘There is only one God and the prophet Mohammed is his messenger.”

Manchester police said they were working to determine whether Salman Abedi worked alone or with help when he bombed the Ariana Grande concert. Some security officials speculated that the bomb was too sophisticated for the one bomber to have created on his own, according to a BBC News report.

“I think it’s very clear that this is a network that we are investigating,” Manchester police chief Ian Hopkins told reporters Wednesday. “And as I’ve said, it continues at a pace. There’s extensive investigations going on and activity taking place across Greater Manchester as we speak.”

The United Kingdom’s domestic security service escalated the national terror threat level Tuesday from severe to critical. Critical is the highest possible threat level and means an “attack is expected imminently,” according to MI5 guidelines. Thousands of soldiers were set to be deployed on streets and at critical venues to prevent another attack.

Authorities said they were also attempting to determine if Salman Abedi had any ties to Raphael Hostey, a British recruiter for ISIS who died last year during a drone strike, according to the New York Times.

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A still image from a video shows people fleeing the Manchester Arena following an attack in Manchester, United Kingdom, May 22, 2017. Reuters