U.K. Home Secretary Amber Rudd said Facebook’s WhatsApp messaging system should open its encryption to security services and urged online companies to be more aggressive in shutting down sites exploited by terrorists.

After newspapers disclosed that Khalid Masood, who killed four people in London last week, had used ­WhatsApp shortly before he began his attack, Rudd identified the company as needing to do more to help fight terrorism.

“It’s completely unacceptable” that messages can’t be opened, Rudd told the BBC on Sunday. “We need to make sure that our intelligence services have the ability to get into encrypted services like WhatsApp.”

Since the attack, government ministers have berated online companies for taking inadequate steps to stop the spread of hate messages. Writing in the Sunday Telegraph, Rudd said the internet is “serving as a conduit, ­inciting and inspiring violence, and spreading extremist ideology.” ­Foreign Secretary Boris Johnson told the Sunday Times that “They need to stop just making money out of prurient ­violent material.”

Rudd said she has asked executives from the internet companies to a meeting this week. “They’re going to get a lot more than a ticking off,” she said.

Facebook bought WhatsApp for $22 billion in 2014. A spokesperson for WhatsApp said, “we are horrified by the attack carried out in London earlier this week and are ­cooperating with law enforcement as they continue their investigations.”

Governments and security agencies face an uphill struggle to keep up with new technology, and while the FBI has managed to unlock iPhones in order to obtain data, messaging tools are harder to crack.

Encryption scrambles data using a proprietary code that can only be unlocked with a special key. In April 2016, WhatsApp gave its users encryption by default, as well as complete control over the keys for all its messaging services, including photos, phone calls and group chats. Apple began offering full end-to-end encryption for its iMessage platform and FaceTime video service about five years ago.

“When you send a message, the only person who can read it is the person or group chat that you send that message to,” WhatsApp co-founders Jan Koum and Brian Acton wrote in a blog post following the launch of its encryption service last year. “No one can see inside that message. Not cybercriminals. Not hackers. Not oppressive regimes. Not even us.”

Some governments have taken overt measures in their attempts to obtain data. ­WhatsApp was temporarily blocked in Brazil in December 2015, after the company refused to allow the government to see communications among drug dealers involved in a criminal case.

However, tech companies have presented a united front against attempts from governments to obtain data. WhatsApp publicly backed Apple in its argument with the FBI regarding the unlocking of an iPhone that belonged to one of the shooters in a December 2015 massacre in San ­Bernardino, Calif.