"This is clearly a network that we are investigating, and extensive activity is taking place across Greater Manchester as we speak," Chief Constable Ian Hopkins of the Greater Manchester Police said today.
Five arrests have been made in connection to the attack at the Ariana Grande concert. Twenty-two people, including a police officer, were killed.
PHOTOS: Terror attack at Ariana Grande concert in Manchester
The Greater Manchester Police said today a fifth person was arrested in Wigan, located about 25 miles west of Manchester.
"When arrested, the man was carrying a package which we are currently assessing," the police department tweeted. "A police cordon remains in place while this is carried out."
On Tuesday, U.K. Prime Minister Theresa May raised the country's threat level to critical - the highest of the United Kingdom's five threat levels - indicating that another attack may be imminent.
Investigators on Tuesday identified 22-year-old Salman Abedi as the suspected suicide bomber. He died at the scene.
ISIS has claimed responsibility for the bombing.
Officials said that 119 people went to hospitals after the bombing, 64 of them are being treated and 20 of those are in critical condition.
RELATED: Victims identified in deadly Manchester concert attack
Saffie-Rose Roussos, 8, is the youngest known person who died from the attack. She was separated from her mother and sister, who were among the wounded, police said.
The mother of 15-year-old Olivia Campbell confirmed on Facebook that her daughter was among those killed.
Runshaw College in Lancashire confirmed on its Facebook page that 18-year-old student Georgina Bethany Callander died from the attack.
Lisa Lees, a 43-year-old mother and grandmother, was confirmed to be among the dead by her daughter, Lauren Ashleigh Lees.
"My mum was an amazing lady and wife," her daughter said in a statement. "We will pull together as a family and help each other through the darkness."
May said Tuesday, "We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but as an opportunity for carnage."
Witness Joseph Harries told ABC News' "Good Morning America" that "people were just trying to get out of the arena as fast as they possibly could after the blast. I was directly in front of the stage at the heart of the arena. I had exactly the same distance to get out of any of the doors."
"I had my best friend with me, and I grabbed hold of her wrist and told her never let go of me," he said. "We just ran. We jumped over chairs, railings to get out of the doors. We had to force open doors that wouldn't open because people were trying to get to - the entire capacity of the 20,000-person arena were trying to get out of one exit."
"It felt like an eternity," Harries said, but it "couldn't have been more than two, three minutes from in our seats to outside of the arena."
On Wednesday the Metropolitan Police Service in London said that the public should expect to "see more armed officers" patrolling the streets in response to the elevated terrorist threat levels.
"The reality is that we must be prepared to be able to respond to and deal with armed and deadly attackers, so we must be in a position to respond with firearms officers who will use force to stop those attackers in their aim," Metropolitan Police Service Commander Jane Connors said in a statement. "That is why is we have increased the number of firearms who are on duty, both out walking and in roaming patrols, at fixed points and carrying out a range of operations."
Amid the security developments, the U.K. Ministry of Defense said the Changing the Guard ceremony at Buckingham Palace has been canceled for Wednesday in order to allow police officers to be redeployed.
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