What more do we know about the investigation? ITV News Global Security Editor Rohit Kachroo explains
The name of the suspect being questioned following the stabbing of Sir David Amess MP is Ali Harbi Ali.
The man, who is understood to be a 25-year-old British citizen, was arrested after the Southend West MP was stabbed at a constituency surgery on Friday.
Detectives have until Friday to question him in custody after they were granted a warrant of further detention.
Three properties were seen being searched by police between Saturday night and Sunday morning.
In a statement, Scotland Yard said: “Late on Friday, whilst in police custody, the man was subsequently further detained under section 41 of the Terrorism Act 2000 and he is now being held at a London police station.
“On Saturday, detectives were granted a warrant of further detention at Westminster Magistrates’ Court, allowing them to keep the man in custody until October 22, when the warrant expires.
“As part of the fast-paced investigation, officers have attended three addresses in the London area and conducted searches.
"One of these searches has concluded and the others are ongoing. A post-mortem examination has taken place today.”