President Donald Trump has rebuked the FBI for missing signals before Wednesday's school shooting in Florida.
The agency was "spending too much time trying to prove Russian collusion with the Trump campaign", Mr Trump tweeted.
"There is no collusion. Get back to the basics and make us all proud."
The FBI has admitted it failed to act on a tip-off about suspected shooter Nikolas Cruz, who has confessed to the shooting at a high school in Parkland in which 17 people died.
It was the deadliest US school shooting since 2012 and has re-ignited long-running debates about tougher firearm restrictions.
Earlier, students who survived the shooting rallied in Florida, demanding tighter legislation on gun control and criticising the president for receiving financial support from the National Rifle Association (NRA) during his presidential campaign.
In his tweet, he wrote: "Very sad that the FBI missed all of the many signals sent out by the Florida school shooter.This is not acceptable."
He appeared to link the agency's failures in the specific case to the time it has spent investigating possible collusion between Russia and the Trump team during the 2016 presidential campaign.
He has repeatedly denied any links with Moscow.