Second suspect, age 21, arrested in connection with London train explosion

Sept. 17 (UPI) -- A 21-year-old man was arrested in Hounslow, west London, Saturday night in connection with Friday's bombing of a London Underground train.

"He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody," CNN quoted British police as saying in a statement Sunday.

The man was not identified. He is the second person taken into police custody for allegedly playing a role in the morning rush-hour blast at Parsons Green station.

Police also arrested an unidentified 18-year-old suspect in the port town of Dover earlier Saturday.

Both men are being held on suspicion of terror offenses, the BBC said.

Home Secretary Amber Rudd told the British broadcaster the second arrest suggests the attacker was not "a lone wolf," however she also said there is "no evidence" to suggest ISIS was behind the incident.

"But as this unfolds and as we do our investigations, we will make sure we find out how he was radicalized if we can," Rudd said.

Detectives have been interviewing witnesses; reviewing CCTV footage, and videos and photos contributed by the public; and searching a house in Surrey where the 18-year-old is believed to have lived.

British government officials raised the terror threat level from "severe" to "critical" after the incident, which caused a fire that swept through the crowded subway car.

But on Sunday, officials downgraded the threat to "severe," meaning an attack is highly likely. "Critical" means an attack is imminent.