WESTMINSTER Bridge attacker Khalid Masood sent a WhatsApp message that cannot be accessed because it was encrypted by the popular messaging service, a top British security official said yesterday.

British press reports suggest Masood used the easily available messaging service just minutes before starting a rampage on Wednesday that left three pedestrians and one police officer dead and dozens more wounded, including some with catastrophic injuries.

Home Secretary Amber Rudd used appearances on BBC and Sky News to urge WhatsApp and other encrypted services to make their platforms accessible to intelligence services and police trying to carry out lawful eavesdropping.

“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. In the United States, Apple fought the FBI’s request for the passcodes needed to unlock an iPhone that had been used by one of the perpetrators in the 2015 extremist attack on San Bernardino, California.

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 on 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.