Transcript for New details in the investigation of the London terror attack

at least 100 people were killed. We're learning more about the final moments of the man accused in the horrific attack in London. Details coming out with ties to Saudi Arabia, and questions intensifying on whether he worked alone. ABC's Jennifer Eccleston with the latest from our London bureau. Reporter: New video of the attacker moments after his deadly rampage. Police rush to the policeman fatally stabbed. The killer already shot down, surrounded. The Gates into Britain's parliament open, unguarded. This image shows that policeman posing for a photograph with an American tourist moments before that horrific scene. Tonight, investigators are racing to unravel Khalid Masood's past. Saudi Arabia confirming Masood was in their country three times since 2005. Stressing he was not on a terror or criminal watch list. Also tonight, that key question. Did he act alone? British media reporting the 52-year-old used the whatsapp messaging service two minutes before the terror unfolded. Just who was he communicating with? Not known. The British Muslim convert was born Adrian Russell ajao. Spending time in prison for assault and investigated for links with extremists more than a decade ago. He was determined low risk. Ten men have been arrested and released since the rampage, and police will be searching computer files seized for any indication that the perpetrator received support, encouragement or directions to carry out the attack. Jennifer, thank you. There is much more on "World

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