Anti-terror police have arrested a 30-year-old man in connection with the terror attack in Westminster that killed four innocent people.
The man was arrested on Sunday at an address in Birmingham on suspicion of the preparation of terrorist acts.
He remains in police custody.
Police said another 58-year-old man, arrested on Thursday, also remains in police custody.
A 32-year-old woman was arrested on 24 March on suspicion of terror-related offences and has been released on bail.
Officers have arrested a total of 12 people following a huge investigation into Wednesday’s attack.
Nine have been released with no further action.
This latest arrest comes after Scotland Yard released a statement saying it believed Masood had acted alone during his murderous rampage through central London.
In an attack lasting just 82 seconds, Masood killed four people and injured more than 50 before being shot dead by police.
Deputy assistant commissioner Neil Basu said on Saturday: "We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned.
"Even if he acted alone in the preparation we need to establish with absolute clarity why he did these unspeakable acts to bring reassurance to Londoners, and to provide answers and closure for the families of those killed and the victims and survivors of this atrocity.
"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him."
Specialist police teams turned up at the address in Hockley – just yards from where Masood had previously lived - on Saturday lunchtime.
Masood, 52, from Birmingham, had until the end of 2016 lived in Quayside, less than 200 yards away.
Shocked neighbours told reporters that friends of the attacker lived at the maisonette and revealed Masood would often visit to take their children to a local mosque.
The neighbours said they had not seen Masood or his children since his family left before Christmas.
Masood killed three pedestrians in his rampage, during which he mowed through people walking across on Westminster Bridge.
He then launched a knife attack on serving Met officer Pc Palmer, who later died at the scene.
Masood was then shot down and killed by armed police.
Masood's victims on the bridge included US tourist Kurt Cochran and his wife Melissa, from Utah, who were on the last day of a trip celebrating their 25th wedding anniversary. Mr Cochran was killed and Mrs Cochran was badly injured.
Aysha Frade, who worked in administration at independent sixth-form school DLD College London, in Westminster, also died. She is believed to be a 43 year-old married mother of two.
Leslie Rhodes, a retired window cleaner from Clapham, south London, died from injuries sustained in the attack. He was described by neighbours as a "lovely man".