Mr Biden has ordered a full investigation into the failings, which is running concurrently with the FBI investigation into the shooting itself.
Investigators said Crooks appeared to be worried about his mental health, searching for “major depressive disorder” before carrying out the attack.
His former classmates have described him as a reserved student who was bullied by his peers.
Vincent Taormina, who knew Crooks at school in 2016, told Fox News he was “smug” about politics and had mocked him for supporting Trump.
“He just did not like politicians, especially with the choices that we had,” he said. “He did not like our politicians.”
“I brought up the fact that I’m Hispanic and, you know, I’m for Trump. And he said, ‘Well, you’re Hispanic, so shouldn’t you hate Trump?’
“He called me stupid – or insinuated that I was stupid.”