The battle over encryption, and the role tech companies should play in criminal investigations, has re-emerged after U.K. investigators discovered that Khalid Masood, the alleged perpetrator of last week's terror attack in London, used WhatsApp before the attack.

U.K. government officials are calling for WhatsApp, and other encrypted chat apps, to create a backdoor so law enforcement agencies can access user data and read encrypted messages. The new call for tech companies to grant the government access to encrypted data comes after investigators were unable to read Masood's messages on WhatsApp, which are protected by end-to-end encryption.

U.K. home secretary Amber Rudd, during an interview on the BBC's Andrew Marr Show on Sunday, said it is "completely unacceptable" that the authorities cannot read Masood's messages. "There should be no place for terrorists to hide," Rudd said, explaining how she believes national security supersedes concerns over privacy.

Rudd also told Marr that she has invited tech executives to meet this Thursday, March 30, to discuss how tech companies can take "more responsibility" to help the government when terrorists use their technology.

"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," Rudd said.

In a statement, WhatsApp said the company was "horrified" by the terror attack in front of Parliament, which left four dead and dozens injured, and that it was "cooperating with law enforcement," The Guardian reported.

WhatsApp, which is owned by Facebook, says it cannot give the government access to user data because the company does not store messages on its servers. Messages are stored on a user's phone and protected with encryption.

In December 2015, Federal Bureau of Investigations demanded that Apple unlock the iPhone belonging to one of the terrorists allegedly involved in the San Bernardino shootings. Apple refused the FBI's request and fought a court order demanding it unlock the phone. Eventually, the FBI broke into the phone without Apple's help by hiring a third-party security company.

WhatsApp and Facebook, as well as other tech companies, have a big financial interest in protecting user data from the government. U.S. tech companies lost billions of dollars after Edward Snowden revealed how the U.S. intelligence agencies maintained backdoors into U.S. cloud services.

In a letter to customers last year, Apple CEO Tim Cook defended the company's decision not to cooperate with the FBI's demands.

"While we believe the FBI's intentions are good, it would be wrong for the government to force us to build a backdoor into our products," Cook wrote. "And ultimately, we fear that this demand would undermine the very freedoms and liberty our government is meant to protect."