London, March 24 (RHC-BBC) -- Police in London are appealing for information as they try to establish whether Westminster attacker Khalid Masood acted alone or was directed by others.

Assistant Deputy Commissioner Mark Rowley, of the Metropolitan Police, said the investigation would focus on Masood's "motivation, preparation and his associates," as the police tries to establish whether the perpetrator acted alone inspired by terrorist propaganda, or had been encouraged, supported or directed by others.

Fifty people were injured in Wednesday's attack, with 31 receiving hospital treatment. Two are in a critical condition, and one has life-threatening injuries. Two officers also remain in hospital with serious injuries. Police officer Keith Palmer, 48, died after he was stabbed by Masood.

Masood, 52, had driven his car onto the pavement and into pedestrians on Westminster Bridge, before crashing the vehicle into railings and running into the grounds of Parliament. Armed with a knife, he stabbed PC Palmer, before being shot dead by police.