LONDON • American tourist Staci Martin took a photograph with a police officer standing guard outside the Houses of Parliament in London because she was intrigued by his hat.

Little did she know then that it would be one of the last photos of him alive, as Mr Keith Palmer died hours later, stabbed while trying to apprehend a terrorist who had charged into the Parliamentary compound. "It's my first time in London and I see his hat and I'm like, 'I have to take a picture of him with his hat'," she told ABC News.

"I walked up to him and said 'Do you mind if I take a picture?' He said 'No problem', he was really nice."

Ms Martin told CNN she saw the news about the attack in her hotel room later in the day, and wondered if the officer she took the photo with was safe.

"We saw the picture on the news and thought it might be the same officer. We zoomed in on the photo and realised it was Palmer," said Ms Martin, who lives in Florida. "I want to get this picture to his family and I want people to know what a nice person he was and how fragile life can be."

Mr Palmer, a father of two, was one of four people killed in the attack by Khalid Masood last Wednesday. Masood was shot dead by another officer.