The danger on Friday was limited as Congress was in recess and relatively few people were in the building.
However, thousands of troops have remained around the Capitol building to guard against further attacks the Federal Bureau of Investigation had warned of.
Flags will be flown at half-staff until sunset Tuesday.
Some Republican lawmakers had taken to calling the extensive fencing after the January 6 riot "Fort Pelosi". "All three have been transported to the hospital", the US Capitol Police said on Twitter.
It doesn't appear the driver was known to Capitol or Metro Police, officials said.
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Rep. Lauren Boebert, R-Colo., questioned whether the Federal Bureau of Investigation and Department of Homeland Security were - or should have been - aware that the suspect who attacked Capitol Hill on Friday may have been a threat, or whether the agencies had been distracted by "woke training".
But Washington Metropolitan Police Chief Robert Contee indicated that a terror link was not suspected at this stage.
Five people died in the January 6 riot, including Capitol Police officer Brian Sicknick, who was among a badly outnumbered force trying to fight off insurrectionists seeking to overturn the election.
At the time Trump's supporters shouted slogans including "Stop the Steal" and "Hang Mike Pence" as they attacked the Capitol and said they hoped to stop the election certification.
In recent weeks some security has been loosened, with the number of armed National Guard troops at the Capitol reduced and a security fence that created a broad perimeter around the Capitol complex removed.
"I am heartbroken. Please join me in praying for Officer Evans' family and all of our @CapitolPolice officers amid this tragedy", Miller said.
The entry point on Constitution Avenue where the vehicle struck the barricade is directly where Senators and their staff come to the Capitol each day, according to observers.
Senate Majority Leader Chuck Schumer (D-N.Y.), who was back home in Brooklyn when the chaos unfolded, said he was devastated by yet another tragedy at his workplace. "I'm heartbroken to learn of an officer's death in today's attack", tweeted Representative Angie Craig. "Grateful to all the USCP and first responders who are on the scene".