The Manchester terror attack has already claimed the lives of 22 people and around 60 more people have been seriously injured following the explosion at the Ariana Grande concert at the Manchester Arena.
The bomber then went to the ticket area and detonated the explosive when people began to leave the concert, the report said, citing sources.
"Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn't answering her phone, pls help me", said one message posted alongside a picture of a blonde girl with flowers in her hair.
Teenage screams filled the arena just after the explosion, which also killed the attacker and injured dozens. James said he believed he heard a second explosion as they were running out.
"The priority is to establish whether he was acting alone or as part of a network", Hopkins said.
Speaking on the steps of Downing Street afterwards, Mrs May said: "It is the worst attack [Manchester] has experienced and the worst-ever to hit the north of England".
The police departments said they were monitoring developments with various agencies, including the Federal Bureau of Investigation.
The initial death toll had been put at 19.
The leader of Britain's Labour Party, Jeremy Corbyn, offered support on Twitter.
The local ambulance service said on Twitter it had taken 59 casualties from the incident and treated "a number of walking wounded on scene".
The attack took place after the conclusion of Monday's Ariana Grande concert at Manchester Arena.
"I would like to pay tribute to the bravery and dedication of the emergency services".
"My deepest sympathies are with the victims, and with families who have lost loved ones, as well as those desperately waiting for news", Farron said.
The news reporter went viral for the tasteless jokes in the wake of Monday's attack, which was described by officials in the U.K.as a suicide bombing.
53Two, an exhibition venue some 500 metres (yards) south of the blast site, said it would keep its doors open late.
Andy James, a witness, said he heard an explosion and ran out of the arena with 9-year-old brother in his arms. Grande herself is "okay," one of her representatives told Variety.
She shared gruesome pictures of her stained handbag and bloody hair, writing: "I understand these images might be upsetting, however, I feel as though people need to be shown just how cruel this world really is". "I looked across the arena and everyone over there started running in different directions, screaming in panic", she said.
"Traumatized concertgoers, including children separated from parents, screamed and fled in what appeared to be the deadliest episode of terrorism in Britain since the 2005 London subway bombings".
Manchester Arena, formerly known as the MEN Arena, is the biggest indoor venue in the city with a capacity of around 18,000 for concerts.
British Prime Minister Theresa May is due to chair a meeting of the government's COBRA emergency committee later Tuesday.
BBC said senior counterterrorism officers were assembling in London and liaising with the Home Office.