LONDON: Police stepped up their investigation Monday into the bombing of a packed London Underground train during rush hour after officers made a second arrest in their probe. “The Joint Terrorist Analysis Center, which reviews the threat level that the UK is under, have decided to lower that level from critical to severe,” interior minister Amber Rudd said in a televised statement. A critical threat level means another attack is “expected imminently” while a severe threat indicates an assault is highly likely. Police said earlier Sunday that a 21-year-old man, who has not been identified, was detained late Saturday in Hounslow. A search was underway on Sunday in Stanwell, a few miles west of Hounslow, in connection with the 21-year-old’s arrest, police said. After taking into custody an 18-year-old man earlier on Saturday over the “bucket bomb” attack at the Parsons Green Underground station, police said they were hunting for more suspects. The bomb went off on Friday’s morning in a crowded carriage and although the device is thought to have malfunctioned, it still wounded 30 people. It was the country’s fifth terror attack in six months, a series that has claimed 35 lives.