British security services are pleading with the creators of a popular messaging service for help accessing one of the final communications sent by Westminster Bridge terror attacker Khalid Masood before his deadly assault on Wednesday.
Masood sent an encrypted message on WhatsApp just minutes before he began the rampage that killed three pedestrians and a police officer and wounded dozens of others.
Home Secretary Amber Rudd on Sunday urged those behind WhatsApp -- and similar apps -- to make their platforms accessible to intelligence services.
"We need to make sure that organizations like WhatsApp — and there are plenty of others like that — don't provide a secret place for terrorists to communicate with each other," she said.
Rudd did not provide any details about Masood's use of WhatsApp, saying only "this terrorist sent a WhatsApp message and it can't be accessed."
But her call for a "back door" system to allow authorities to access information is likely to be met with resistance throughout the industry.
The British plea mirrors one made by the FBI following the San Bernardino terror attack in December 2015. After that incident, investigators asked Apple -- to no avail -- to help unlock one of the terrorist's iPhones.
Eventually, the FBI found its own way into the device.
Masood drove a rented SUV into pedestrians on Westminster Bridge before smashing it into Parliament's gates and rushing onto the grounds, where he stabbed a policeman to death before he was shot dead. A detailed police reconstruction has found the entire attack lasted 82 seconds.
Police say he acted alone but they are trying to pinpoint his motive and identify any possible accomplices, making the WhatsApp message a potential clue to his state of mind and his social media contacts.
Rudd said attacks like Masood's would be easier to prevent if authorities could penetrate encrypted services after obtaining a warrant similar to the ones used to listen in on telephone calls or — in snail mail days — steam open letters and read their contents.
Without a change in the system, she said terrorists would be able to communicate with each other without fear of being overheard even in cases where a legal warrant has been obtained.
Rudd also urged technology companies to do a better job at preventing the publication of material that promotes extremism. She plans to meet with firms Thursday in a bid to set up an industry board that would take steps to make the web less useful to extremists.
British police investigating the attack say they still believe Masood, a 52-year-old Briton, acted alone and say they have no indications that further attacks are planned.
Deputy Assistant Commissioner Neil Basu said it may never be possible to fully determine Masood's motives.
"That understanding may have died with him," Basu said Saturday night as police appealed for people who knew Masood or saw him to contact investigators. "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."
ISIS, which is losing territory in Iraq and Syria but still has radical followers in other parts of the world, has claimed Masood was a "soldier" carrying out its wishes to attack Western countries. The group has been known to communicate with followers on WhatsApp, as well as other messaging apps.
Masood had convictions for violent crimes in the U.K. and spent time in prison. He also worked in Saudi Arabia teaching English for two years and traveled there again in 2015 on a visa designed for religious pilgrimages.
One 58-year-old man remains in custody in the case after being arrested in Birmingham, where Masood had been living. He has not been charged or named. Nine others arrested after the assault have been freed without charges and one has been freed on bail.
The family of slain police officer Keith Palmer, meanwhile, released a statement thanking those who tried to save his life.
"There was nothing more you could have done. You did your best and we are just grateful he was not alone," the statement said.
The Associated Press contributed to this report.