The U.K. government is pushing for backdoor access to encrypted messaging apps after it emerged the killer in last week's attack in London used WhatsApp just minutes before striking.

Home Secretary Amber Rudd said WhatsApp -- owned by Facebook (FB) -- and other services cannot provide "a secret place for terrorists to communicate with each other."

"It used to be that people would steam-open envelopes or just listen in on phones when they wanted to find out what people were doing, legally, through warranty," Rudd said in a BBC interview on Sunday. "But on this situation we need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."

Facebook did not immediately respond to a request for comment.

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Government officials confirmed that they have invited tech companies to a meeting on Thursday to talk about encrypted messaging, as well as extremist content on Facebook and Google (GOOGL).

The search giant is already under fire for allowing advertisements to appear alongside extremist content. Several big clients, including the British government, have suspended their ad spending with Google in the last two weeks.

WhatsApp added full end-to-end encryption for all communications in April 2016. It said then that it was impossible for third parties -- including the company itself -- to peek into users' encrypted conversations.

Related: Telegram founder: Opening 'back door' to encrypted apps could aid terrorists

Rudd raised the issue with representatives of other European governments at a meeting Monday in Brussels to discuss the EU's approach to cybersecurity.

Carmelo Abela, who chaired the meeting, wouldn't directly support Rudd's call for companies to provide a backdoor into their apps but he agreed the matter needed further discussion.

"I think there is a fine line here, we need to protect the privacy of the people but we also need to protect the security of the people," Abela said.

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Some experts say Rudd's demands are a non-starter.

"Even if the U.K. government were able to convince WhatsApp to grant them access -- they could not give them access the way the messenger is written today, they would have to recode the app," Internet security researcher Andrea Stroppa said.

That would allow anyone -- not just security services -- the chance to snoop on messages, he said.

The U.K. demands come just a year after a bitter public fight between the FBI and Apple (AAPL) over encryption.

The U.S. Department of Justice asked the company to unlock the iPhone of one of the terrorists involved in the San Bernardino shootings in December 2015.

Apple refused the request and fought a court order ordering it to comply. The FBI eventually dropped the case after it managed to get into the iPhone with the help of an unidentified third party.

Stroppa said that showed intelligence agencies can "hack" their way into communications when they need to, without forcing companies to provide a backdoor to their technology.