London subway blast

Armed police patrol in the Westminster subway station on Sept. 16, 2017, in London.

An armed police officer patrols in Horse Guards Parade on Sept. 16, 2017, in London.

An armed police officer and a member of the military stand guard as the Household Cavalry leave the Horse Guards Parade in central London on Sept. 16, 2017.

A police forensic officer stands beside the subway train where an explosion happened on Sept. 15, 2017, that London police say they are investigating as a terrorist attack.

Armed British police officers walk through the carriage of a London underground tube carriage at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station.

The platform at Parsons Green station in west London on Sept. 15, 2017.

An injured woman is assisted by a police officer close to Parsons Green station in west London after an explosion on a packed London subway train on Sept. 15, 2017.

A woman reacts as she talks on a mobile phone neatr to Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station.

Police forensic officers walk within a cordon near where an explosion occurred at Parsons Green subway station in London on Sept. 15, 2017.

Metropolitan Police Assistant Commissioner Mark Rowley arrives at 10 Downing Street in London following a terrorist attack at Parsons Green subway station on Sept. 15, 2017.

An armed British police officer works near Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an “incident” at Parsons Green underground station in west London on Friday, following media reports of an explosion.

Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an “incident” at Parsons Green underground station in west London on Friday, following media reports of an explosion.

Emergency personnel outside Parsons Green station in west London after a terrorist attack on Sept. 15, 2017.

Members of the emergency services work near Parsons Green underground tube station in west London on Sept. 15, 2017, following an incident on an underground tube carriage at the station.

Members of the emergency services work outside Parsons Green underground tube station in west London following an incident on an underground tube carriage at the station on on Sept. 15, 2017,.

Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station.