The US Justice Department will investigate the police response to the mass shooting at a school in Uvalde, Texas, which killed 19 children and two teachers.
Public anger is growing after it emerged that officers waited in the hallway as children trapped with the shooter made desperate 911 calls.
US President Joe Biden is in Uvalde to meet families devastated by the attack.
He will also meet survivors and first responders.
First Lady Jill Biden, herself a teacher, accompanied the president to a memorial at Robb Elementary School. They were seen comforting the school's principal Mandy Gutierrez, beside a carpet of floral tributes for the teachers, and children - all under 10 years old - who lost their lives.
Both were seen wiping tears from beneath their sunglasses. Mrs Biden touched each child's photo in turn.