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Five people, including a London police officer who was stabbed and the alleged assailant, were killed in a terror attack that saw more 40 people injured outside the Parliament building on Wednesday in an act described as "sick and depraved" by British Prime Minister Theresa May, Fox News reports.

Acting Metropolian Police Deputy Commissioner and Head of Counter Terrorism Mark Rowley said there was only one attacker who authorities believe was "inspired by international terrorism."

Rowley said three civilians were among those killed. He identified the officer killed in the attack as 48-year-old Keith Palmer.

"One of those who died was a police officer from our Parliamentary and Diplomatic Protection Command who had 15 years service," he said.

Rowley said it was "too early" to publicly release the name of the suspect in the attack, but that officials "think we know who the attacker is and are working to establish who his associates are."

Armed and unarmed patrols have been stepped up as a precaution across the country, according to Rowley.

A full counter-terrorism investigation is underway.

The source mentions that no terror group has claimed responsibility for Wednesday's attack yet, but a European government official told Reuters that investigators are considering the possibility the attack was inspired by ISIS propaganda.