Updated at 11:45 a.m. ET
Police officers have arrested three more men as part of the investigation into Monday's bombing at an Ariana Grande concert in Manchester, England. The arrests came after Prime Minister Theresa May raised the terror threat level to "critical," the highest setting.
The identities of those men and the charges against them were not announced. They were arrested "after police executed warrants in South Manchester," the Greater Manchester Police department says. That's the same area of the city where suspected suicide bomber Salman Abedi, 22, is believed to have lived.
Abedi, who was reportedly born in the U.K. to Libyan parents, is believed to have traveled to Libya recently, according to multiple media outlets that cite U.S. officials with knowledge of the investigation. Repeatedly, British officials have said that they're focused on finding out whether the attacker acted alone or with the support of a larger group.
"I think it's very clear that this is a network that we are investigating," Greater Manchester Police Chief Ian Hopkins said in an update on the case Wednesday.
From Manchester, NPR's Frank Langfitt reports:
"In some of my conversations that I've had with people who are following this very closely, the feeling is that they were aware of the bomber, but they didn't think that he had the ability to pull off something like this, something of such sophistication and something so effective and deadly."
In Monday night's attack, at least 22 people died and nearly 60 were sent to hospitals after an explosion struck in the area between the concert venue and the adjacent transit station. As of Wednesday morning, police say, they've identified all of the fatalities and contacted all of the families involved.
Those killed in the attack, which targeted a concert by a pop singer whose core fan group is young girls, include 8-year-old Saffie Rose Roussos. Other victims who've been identified range from teenagers to a Polish couple, Angelika and Marcin Klis, who came to the venue to pick up their daughters.
Wednesday's announcement brings to four the number of attack-related arrests to four, following an arrest announced Tuesday. ISIS has issued a claim of responsibility, but the level of the terrorist group's involvement isn't known.
The U.K. is now at its highest terror threat level for the first time since 2007. As part of the security response, military personnel are being deployed into civilian areas to work alongside and bolster police.
The head of the U.K.'s National Counter Terrorism Policing unit, Assistant Commissioner Mark Rowley, says that using military troops "is part of an agreed and well-rehearsed plan and military personnel will remain under the command and control of the police service."
"The investigation into the terrorist attack in Manchester is large-scale, fast-moving and making good progress," Rowley said.
In London, police and security are being boosted, the Metropolitan Police Service announced Wednesday. Police are asking the public "to remain calm but alert" and to report suspicious behavior.
"We will do all we can to protect the capital that we serve at this unsettling time," said Commander Jane Connors, who leads the London policing operation. "All our work is designed to make our city as hostile an environment as possible for terrorists to plan and operate."
The more prominent security presence in London will include armed officers who are supporting what the police call Project Servator. Here's the description they gave:
"This tactic uses teams of specialist police officers who have been trained to spot the telltale signs that a person may be carrying out hostile reconnaissance or committing other crime. The approach is based on extensive research into the psychology of criminals and what undermines their activities."