"We are forensically examining a complicated crime scene that covers a wide area, and as with all investigations of this nature it will take us some time to work through the painstaking work necessary to get all of the relevant evidence," Metropolitan Police Acting Deputy Commissioner Mark Rowley said as quoted by The Guardian.
He added that some entrances to the parliamentary estate will remain closed.
On Wednesday, an attacker, whose identity is yet to be revealed by police, hit several people while driving on Westminster Bridge. He then tried to enter the parliament armed with a knife. After stabbing a police officer, who was later confirmed dead at the scene, the attacker was shot.
According to Rowley, the Wednesday terror act resulted in the death of five people, including the attacker, who could have been inspired by international terrorism. At least 40 people were injured, including three police officers, two of whom are in serious condition.