A 19-year-old man was charged with murder Friday in the stabbing attack of an American woman that also left five other people injured this week in London's Russell Square.
Zakaria Bulhan was arrested at the scene of the attack Wednesday night and charged with one count of murder and five counts of attempted murder. Police said he will appear in custody Saturday at Westminster Magistrates' Court.
Despite earlier saying terrorism was a line of inquiry, police said Thursday that the attack appeared to have been triggered by mental health issues and was not terror-related.
"Whilst the investigation is not yet complete, all of the work that we have done so far, increasingly points to this tragic incident as having been triggered by mental health issues," Assistant Commissioner Mark Rowley told journalists.
Bulhan, a Norwegian national of Somali origin, was interviewed after being arrested on suspicion of murder, and addresses in London have been searched.
"So far we have found no evidence of radicalisation or anything that would suggest the man in our custody was motivated by terrorism," Rowley said.
He described the attack as "spontaneous" and said the victims appeared to have been selected at random.
The woman who was killed was identified as Darlene Horton, 64. She was married to Florida State University psychology professor Richard Wagner. The couple were in London as Wagner was teaching a summer study abroad program.
The program had recently completed, and the couple had been scheduled to return to the U.S. on Thursday, university officials said.
"There are no words to express our heartache over this horrible tragedy," FSU President John Thrasher said in a statement. "We are shocked that such senseless has touched our FSU family, and we will do all we can to assist Professor Wagner and his loved ones, as well as his friends and colleagues in the Psychology department, as they mourn."
One of the people injured, a British man, remains in hospital after being stabbed in the stomach. His condition was described as serious but stable.
The other four people hurt have been discharged from hospital. They include an American man who was stabbed in the chest, an Australian man and woman who were stabbed in the chest and back respectively, and an Israeli woman who was stabbed in the arm.
Rowley said that based on interviews with the suspect and family members, witness accounts, address searches, and a full intelligence review by the security services, there was nothing to suggest the suspect's background was relevant to the motive for his actions.
Explaining the fact that terrorism was an initial line of inquiry, he said: "You would expect us in the current climate of threat and with the events across Europe, when we have a violent attack like this with multiple victims, to consider all possibilities."
The Metropolitan police had said in a statement issued Thursday that "early indications suggest that mental health is a significant factor in this case, but we retain an open mind regarding the motive and terrorism remains one line of inquiry".
However, in a statement issued just before 9am, all mentions of terror were removed.
A police source told BuzzFeed News at the time that this "spoke volumes about the way this investigation is heading".
Nevertheless, while the investigation is being led by Scotland Yard's homicide command, counterterrorism authorities were assisting.
Police were first called to Russell Square around 10:30pm after reports of a man with a knife, officials said. Six people were found with injuries, and one of them, a woman in her sixties, was later pronounced dead. Forensic units remained at the scene on Thursday morning.
Police said the other five people – two women and three men – received "various injuries" in the attack.
A man was arrested minutes later after an officer used a Taser to subdue him, police said.
In response to the attack, officials said residents should expect to see more police on London streets from Thursday, including armed officers.
"This was a tragic incident resulting in the death of one woman and five others being injured. Early indications suggest that mental health was a factor in this horrific attack. However we are keeping an open mind regarding the motive," Assistant Commissioner Rowley said in statement issued around 9am that removed the line "and terrorism remains one line of inquiry being explored".
His statement continued: "As a precautionary measure, Londoners will wake up this morning to notice an increased presence on the streets of officers, including armed officers today. We would urge the public to remain calm, alert and vigilant."
Mayor of London Sadiq Khan echoed those remarks.
"I urge all Londoners to remain calm and vigilant," he said. "Please report anything suspicious to the police. We all have a vital role to play as eyes and ears for our police and security services and in helping to ensure London is protected."
The stabbings came after authorities had announced on Wednesday a plan to increase the number of armed officers in London.
The Metropolitan police said that the number of armed officers in London will rise from 600 to 2,800 as officials hope to deter potential terrorist attackers, the BBC reported.
Speaking on Thursday, Metropolitan police Commissioner Bernard Hogan-Howe said his thoughts were with the family of the victim, and paid tribute to officers who arrived on the scene within six minutes and used a Taser to detain the suspect.
"Their swift response undoubtedly helped to prevent more people from getting injured," he said.
"Across our capital today you will see more police officers, armed and unarmed, to reassure the public after recent events overseas. We will protect our capital and those who live, work and visit our city."
In his most recent statement, Assistant Commissioner Rowley said: "Our firearms officers bravely rushed to that scene with very little information. They would not have known if this was an act of terrorism or the random attack we now know it be.
"They detained an armed and dangerous man – and resolved it using the minimum necessary force – no shots were fired. We should be proud of them and the British tradition of using the minimum necessary force."
Bulhan will face trial in London in February next year.
The 19-year-old, a Norwegian of Somali origin, is accused of killing Darlene Horton, 64, a retired teacher, in Russell Square. He is also charged with two counts of attempted murder, for attacking and injuring two other people in the same incident.
The defendant is expected to enter a plea on 25 October.