British authorities have arrested a second man in connection with the bombing of a London Underground train on Friday, officials said.
The 21-year-old man was arrested in Hounslow, west London, Saturday night by detectives from the London Metropolitan Police Counter Terrorism Command. The incident at Parsons Green station injured 30 passengers.
"He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody," police said in a statement Sunday.
Two people have been arrested as part of a major manhunt for suspects. Police had arrested an 18-year-old suspect in Dover on Saturday. Dover is major port town about 80 miles southeast of London.
UK Home Secretary Amber Rudd had said "it's much too early to say" whether anyone behind the bombing was previously known to authorities.
Shortly after the bombing, the UK terror threat level was raised to "critical," meaning that another attack is considered to be "imminent."
Police are still searching a residential address west of London in Sunbury-on-Thames, Surrey, in connection with Friday's attack.
In London, an increased number of police officers were patrolling the streets over the weekend, particularly around "crowded places, iconic sites, transport hubs and ports," police said in a statement.
The Parsons Green station, where an improvised device exploded as a train pulled into the platform during Friday's morning rush hour, reopened early Saturday.
Investigators are working around the clock to identify those responsible for the bombing.
Detectives have spoken to 100 witnesses and are reviewing hours of CCTV footage as well as videos and photographs sent by the public, Neil Basu, the senior national coordinator for counterterrorism policing said in a statement on Saturday.
Authorities said they will work to keep in check bomb ingredients like those used in Friday's bombing.
"There is no doubt that this was a serious IED (improvised explosive device) ... so we will have to make sure that we take all steps we can to ensure that the materials that this man was able to collect become more and more difficult to combine together," Rudd said.