The U.S. intelligence official, who has direct knowledge of the investigation, said Abedi, whose family is of Libyan descent, was identified by a bank card found in his pocket at the scene of the explosion after an Ariana Grande concert at Manchester Arena. The identification was confirmed by facial recognition technology, the official said.

Abedi had traveled to Libya within the last 12 months, one of multiple countries he had visited, the official said. And while he had "clear ties to al Qaeda," the official said, Abedi could have also had connections to other groups.

Members of his own family had even informed on him in the past, telling British authorities that he was dangerous, according to the intelligence official.

The U.S. official said Abedi's bomb was "big and sophisticated," using materials hard to obtain in Britain — meaning "it's almost impossible to see he didn't have help."

The official said investigators were concerned that Abedi could have been part of a terrorist cell, the extent and allegiances of which remain unclear. A "follow-on" attack is possible, the official said.

British Prime Minister Theresa May echoed the warning in raising the country's threat level from "severe" to "critical," saying Tuesday that "a further attack may be imminent." Under the order, armed military personnel will replace police guarding "key sights" and events like concerts and sports matches, May said.

And London Mayor Sadiq Khan told residents to expect to see armed officers and military personnel on the streets. "They are there to help our police service to keep us safe and guard key sites," he said.