They are investigating whether he acted alone or was part of a wider network.
The attacker, I can confirm, died at the arena.
The attack is the worst in the United Kingdom since coordinated explosions killed 56 people on the London transport network in July 2005.
The social media posts included heartbreaking pleas for information about missing friends and loved ones. Armed police respond after reports of an explosion at Manchester Arena during an Ariana Grande concert in Manchester, England, Monday, May 22, 2017.
"It was a huge explosion - you could feel it in your chest".
At 10:51 p.m. ET Grande sent a tweet saying, "broken. from the bottom of my heart, i am so sorry. i don't have words".
British Prime Minister Theresa May condemned the "appalling terrorist attack" and suspended her campaign for the general election on June 8 along with chief opposition leader Jeremy Corbyn.
Police believe one man, who has not been named, carried out the attack.
May was due to hold a crisis response meeting. The police say the incident is a "terrorist" attack.
Chief Cabinet Secretary Yoshihide Suga said the government had yet to find any Japanese citizens involved in the attack as of Tuesday morning, adding that Japan would cooperate with the worldwide community and actively continue strengthening measures to cope with terrorism.
They came by Twitter, harnessing social media - so often a site of anonymous derision - as a tool of collective uplift.
Home Secretary Amber Rudd described the attack as "barbaric" and "deliberately targeting some of the most vulnerable in our society - young people, children out at a pop concert".
Twitter accounts affiliated to the jihadists have used hashtags referring to the blast to post celebratory messages, with some users encouraging similar attacks elsewhere.
Several blocks away, on the city's main High Street, is the Holiday Inn Express, where according to early reports online, as many as 50 unaccompanied children - who became separated from their parents during the ensuing commotion - are being cared for by staff and locals. The man was shot dead at the scene.
The U.S. Embassy in London is encouraging Americans in Manchester to contact their families.
If you are concerned whether your loved one is safe following yesterday's explosion at Ariana Grande's concert in Manchester then our guide may be able to help you. "They are being treated at eight hospitals across Greater Manchester".
A spokeswoman said: "After the tragic events at Manchester Arena last night, the team at the Metro Radio Arena are aware of the potential heightened level of threat against venues across the country". The explosion, which happened in the arena's foyer area, sent panicked fans running.
"We ran out", Robinson told Reuters.