British counter terrorism police assumed control of the investigation into the fatal stabbing of Conservative MP Sir David Amess in his constituency on Friday.
Early investigations revealed "a potential motivation linked to Islamist extremism", police said. Sir David, 69, was fatally injured while meeting constituents at Belfairs Methodist Church in Leigh-on-Sea near Southend at midday on Friday.
Senior National Co-ordinator for Counter Terrorism Policing, Deputy Assistant Commissioner Dean Haydon, had formally declared the incident as terrorism.
“As part of the investigation, officers are currently carrying out searches at two addresses in the London area and these are ongoing,” the police statement said. “It is believed that he acted alone, and we are not seeking anyone else in connection with the incident at this time. However, enquiries into the circumstances continue.”
Police arrested a 25-year-old man on suspicion of murder in connection with the incident. He is believed to be a British national with Somali heritage, officials told the Press Association.
Home Secretary Priti Patel called the attack a senseless assault on democracy itself and paid tribute to his "endless passion" for his work. "Questions are rightly being asked about the safety of our country's elected representatives and I will provide updates in due course," Ms Patel said.
A spokesman said she had told police chiefs in a conference call to immediately review security arrangements for MPs following the attack.
Desperate attempts were made to save Sir David's life and an air ambulance was sent to the scene.
Essex Police said officers were called to reports of a stabbing in Leigh-on-Sea just after noon on Friday. They said “a man was arrested and a knife recovered".
“We are not looking for anyone else in connection with the incident and do not believe there is an ongoing threat to the wider public," police said.
The father of five, who had been an MP since 1983, is the third serving MP to be killed since 1990. He was knighted by Queen Elizabeth for his public service in 2015. His murder comes just five years after politician Jo Cox was assassinated outside her constituency office in Birstall, west Yorkshire.
Sir David was the chairman of the All-Party Parliamentary British-Qatar Group, and recently met the country's emir in Doha.
Just hours after his death, the flags at Downing Street were lowered in tribute and Prime Minister Boris Johnson said hearts were full of shock and sadness.
"The reason I think people are so shocked and saddened is above all he was one of the kindest, nicest, most gentle people in politics, and he also had an outstanding record of passing laws to help the most vulnerable," said Mr Johnson.
“David was a man who believed passionately in this country and in its future and we’ve lost today a fine public servant and a much-loved friend and colleague, and our thoughts are very much today with his wife, his children, and his family.”
His murder comes just months after the nation marked the fifth anniversary of Ms Cox's death.
The mother of two, whose sister Kim Leadbeater has since become a politician to take on her mantle, was killed by a right-wing extremist as she also held a surgery for local constituents.
British politicians are protected by armed police when they are inside Parliament, but have no such protection in their constituencies.
The Jo Cox Foundation, set up in her memory, said it was "horrified" to learn of Friday's incident.
Her husband, Brendan Cox, described it as a “cowardly” attack.
"My thoughts and love are with David’s family. They are all that matter now," he said.
"This brings everything back. The pain, the loss, but also how much love the public gave us following the loss of Jo. I hope we can do the same for David now.
“Attacking our elected representatives is an attack on democracy itself.
“There is no excuse, no justification. It is as cowardly as it gets.”
Last year, in a book entitled Ayes & Ears: A Survivor's Guide to Westminster, Sir David reflected on Ms Cox's murder.
“We all make ourselves readily available to our constituents and are often dealing with members of the public who have mental health problems, it could happen to any of us,” Sir David wrote.
Ms Leadbeater recalled how the news had reached her and the concerns it immediately ignited.
“Totally shocked by what has happened to think that something so horrific could happen again to another MP, to another family," she said.
“And scared and frightened – a real rollercoaster of emotions.
“My phone started going straight away, my mum and dad, my partner, my friends, ‘Are you OK?’ and I was OK, I was visiting a school.
“But the shock and the feelings for us as a family, obviously what we went through and another family are going through that again, it’s horrific."