U.S. law enforcement officers on heightened alert

ATLANTA - Investigators hunted Tuesday for possible accomplices of the suicide bomber whose blast at an Ariana Grande concert in Manchester killed 22 people, including an 8-year-old girl.

Britain's prime minister warned another attack could be "imminent," and raised the national threat level from severe to critical.

The Islamic State group claimed responsibility for the horror, which also wounded 59 people, though a top American intelligence official said the assertion, could not be verified.

Meanwhile, U.S. law enforcement officers are on heightened alert.  Lt. Theosie Williams is the Homeland Security Commander for Atlanta Police and said while the city plans and trains for events well in advance, when attacks like this happen, they re-group.

“Sleepless nights,” said Lt. Williams.  “You do not know how they are going to attack you if they are going to attack you, did you prepare properly, is your plan put together well enough to prevent that or minimize it.”

Lt. Williams said additional uniform and undercover officers would be deployed to venues across the city. At a concert Tuesday night at the Fox Theatre, Lt. Williams said additional officers were in place.

The city’s video integration center is essential to Atlanta’s level of security, allowing authorities to have eyes in places that officers cannot be at that particular time.

While Atlanta police work with state and federal agencies when threats like this happen, Lt. Williams said the community also plays a role, asking anyone who sees something to say something.

“If they have any suspicion about anything, a hunch is very important to us, so do not minimize anything that you may sense,” said Lt. Williams.

RELATED: Warning of "imminent" attack, Britain raises threat level