Florida school shooting suspect Nikolas Cruz has been charged with 17 counts of premeditated murder.
The 19-year-old was questioned for hours by US federal authorities in relation to yesterday's deadly massacre.
A charge sheet published online today confirmed his status as "pending trial".
And as Cruz was taken to his cell, US President Donald Trump urged Americans to report "disturbed" people showing signs of "erratic behaviour" to the authorities.
Seventeen people were killed as gunfire erupted at Marjorie Stoneman Douglas High School, Parkland, just before classes ended on Wednesday afternoon.
The shooting is now ranked as the second deadliest act of gun violence on a US public school campus.
Cruz was arrested in a nearby community two miles from the school around an hour after the rampage.
CNN, citing law enforcement sources, said the gunman pulled the fire alarm and shot at staff and students before trying to blend in and leave among fleeing crowds.