Five people, including a London police officer who was stabbed and the alleged assailant, were killed in a terror attack that saw more 40 people injured outside the Parliament building on Wednesday in an act described as "sick and depraved" by British Prime Minister Theresa May.
Acting Metropolian Police Deputy Commissioner and Head of Counter Terrorism Mark Rowley said there was only one attacker who authorities believe was "inspired by international terrorism."
Rowley said three civilians were among those killed. He identified the officer killed in the attack as 48-year-old Keith Palmer.
"One of those who died was a police officer from our Parliamentary and Diplomatic Protection Command who had 15 years service," he said.
Rowley said it was "too early" to publiclly release the name of the suspect in the attack, but that officials "think we know who the attacker is and are working to establish who his associates are."
Armed and unarmed patrols have been stepped up as a precaution across the country, according to Rowley.
A full counter-terrorism investigation is underway.
Police said a vehicle mowed down pedestrians on London's Westminster Bridge, leaving more than a dozen with injuries described as catastrophic.
Rowley said the car then crashed near to Parliament, and one man - armed with a knife - continued the attack and tried to enter Parliament.
The knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain's Parliament, sending the compound into lockdown for hours.
The threat level for international terrorism in the U.K. was already listed at severe, and will remain so, according to May. The attack came on the one-year anniversary of the terror attacks in Brussels in which 32 people were killed and more than 300 injured.
Recent Vehicular Terror Attacks:— Fox News Research (@FoxNewsResearch) March 22, 2017
•Jerusalem, Israel: 1/8/17
•Berlin, Germany: 12/19/16
•Ohio State: 11/28/16
•Nice, France: 7/14/16 https://t.co/CAaDgAgko7
ISIS claimed responsibility for the coordinated suicide bombings last year -- two at the Brussels Airport in Zaventem and one at Maalbeek metro station in central Brussels. The bombings were the deadliest terror attack in Belgium's history.
ISIS has long promoted the use of vehicles and knives in attacks by so-called "lone wolf" terrorists, particularly in Western countries. Senior U.S. officials expected an uptick of terror attacks in Europe as they ramp up the fight to take ISIS capital Raqqa, Fox News is told.
The terrorist group has not claimed responsibility for Wednesday's attack, but a European government official told Reuters that investigators are considering the possibility the attack was inspired by ISIS propaganda.
London police were called to the Parliament building at around 2:40 p.m. local time (10:40 a.m. EST) after reports of a vehicle crashing into a crowd at Westminster Bridge near Britain's parliament.
This is the moment Parliament was suspended as "sounds similar to gunfire" were heard outside pic.twitter.com/to6PPxkNqL— Sky News (@SkyNews) March 22, 2017
The French prime minister said French high school students were among the injured on Wednesday.
Colleen Anderson of St Thomas' Hospital said a woman died after a vehicle apparently hit pedestrians on Westminster Bridge, near Parliament, according to the Press Association.
Anderson said: "There were people across the bridge. There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries."
A woman was also pulled alive from River Thames. The Port of London Authority said the female member of the public was recovered from the river, injured but alive.
Rick Longley told the Press Association that he heard a bang and saw a car plow into pedestrians and come to a crashing stop. Images from the scene showed pedestrians sprawled on the ground, with blood streaming from a woman surrounded by a scattering of postcards.
"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben," he said. "A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw."
The former Polish foreign minister Radoslaw Sikorski posted a video on Twitter that seems to show people lying injured in the road on Westminster Bridge.
Sikorski, a senior fellow at the Harvard Centre for European Studies, says he saw at least five people lying on the ground after being "mown down" by a car.
Sikorski told the BBC he "heard what I thought what I thought was just a collision and then I looked through the window of the taxi and someone down, obviously in great distress.
"Then I saw a second person down, and I started filming, then I saw three more people down, one of them bleeding profusely."
Prime Minister Theresa May was rushed to safety after the attack.
"The thoughts of the PM and the government are with those killed and injured in this appalling incident, and with their families," May's office said in a statement.
In a prime-time address to the nation outside 10 Downing Street, May praised the "exceptional bravery" of police, and thanked them and other emergency services "on behalf of the whole country."
May said it was "no accident" the attacker chose Parliament, which stands for democracy, freedom and rule of law.
The White House condemned the attacks, adding that President Trump is continuing to monitor developments out of London.
Spokesman Sean Spicer said Wednesday Trump spoke with May and said that the White House applauds "the quick response of British police and first responders."
The White House said Trump "pledged the full cooperation and support of the United States Government in responding to the attack and bringing those responsible to justice."
The U.S. State Department said it is closely monitoring the incident and urged Americans in London to avoid the area.
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.
"Our hearts go out to those affected," Toner said.
Home Secretary Amber Rudd said the British government's "top priority" is the security of its people, following the attack.
"The British people will be united in working together to defeat those who would harm our shared values: values of democracy, tolerance and the rule of law," Rudd said in a clip broadcast on Sky News.
"The British people will be united in working together to defeat those who would harm our shared values", says Home Secretary Amber Rudd pic.twitter.com/JnCLbrAdyJ— Sky News (@SkyNews) March 22, 2017
Daily Mail journalist Quentin Letts said he saw a man in black attack a police officer outside Parliament before being shot two or three times as he tried to storm into the House of Commons.
"He had something in his hand, it looked like a stick of some sort, and he was challenged by a couple of policemen in yellow jackets," Letts told the BBC. "And one of the yellow-jacketed policemen fell down and we could see the man in black moving his arm in a way that suggested he was stabbing or striking the yellow-jacketed policeman."
Dennis Burns, who was just entering Parliament for a meeting, told the Press Association he heard a radio message saying an officer had been stabbed. Police and security rushed outside as he was going in.
"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street," he said. "The first stream was around 30 people and the second stream was 70 people. It looked like they were running for their lives."
British security has thwarted some 13 terror plots over the past four years, but the UK has largely been spared major international terror attacks such as the ones seen in Belgium and France.
Fox News' Jennifer Griffin and The Associated Press contributed to this report.