London Police: 4 Dead, Including 1 Police Officer; 20 Injured After Attack |

The head of counterterrorism at London's Metropolitan Police, says four people have died in the terror incident in London, including an attacker and a police officer.

Mark Rowley says some 20 people have been wounded and Parliament was locked down. A search is underway to make certain no other attackers are in the area - though police believe there was only one attacker.

Rowley said the dead policeman was one of the armed officers who guard Parliament. The other victims were on Westminster Bridge.

Rowley says "We are satisfied at this stage that it looks like there was only one attacker. But it would be foolish to be overconfident early on."

Officials say a man with a knife attacked a police officer at Parliament and was shot by officers.

A session of Parliament was suspended after the incident.

The White House says U.S. President Donald Trump has been briefed on the incident.

Trump himself said during a brief appearance Wednesday before reporters at the White House that he was just getting the news. He called it "big news."

Trump's spokesman, Sean Spicer, says the U.S. will continue to monitor the situation and update the president.

The U.S. State Department says it is closely monitoring the incident outside London's Parliament.

Spokesman Mark Toner said Wednesday: "We stand ready to assist in any way the U.K. authorities would find helpful."

He added that the U.S. Embassy in London is closely following the news and stands ready to help any affected Americans.

He said: "Our hearts go out to those affected."

French President Francois Hollande and German Chancellor Angela Merkel have both expressed their support and solidarity with Britain after the attacks.

"We are all concerned with terrorism," Hollande told reporters Wednesday during a visit in Villepinte, outside Paris. "France, which has been struck so hard lately, knows what the British people are suffering today."

Hollande added that countries "must bring all the conditions to answer these attacks" and that "it is clear that it is at the European level, and even beyond that, that we must organize ourselves."

French Interior Minister Matthias Fekl said "it is a high place of democracy that has been attacked" and that France is "obviously ready to help."

Merkel said in a statement Wednesday that she learned "with sorrow" of Wednesday's incident and her thoughts were "with our British friends and all of the people of London," in particular those who were injured.

While the circumstances of the attack were still unclear, "I want to say for Germany and its citizens: we stand firmly and resolutely by Great Britain's side in the fight against all forms of terrorism," she said.