Parkland: The 19-year-old man accused of shooting 17 people to death at a Florida high school legally purchased the assault rifle used in the killings and may have foreshadowed the attack in a social media comment investigated by the FBI last year, authorities said on Thursday.

New details about the troubled background and gun ownership of the suspect, Nikolas Cruz, emerged as he appeared in court to face formal charges of carrying out the second-deadliest mass shooting at a public school in U.S. history.

Authorities also shed more light on how the suspect made a getaway by blending in with students fleeing the school, then casually spent more than an hour drifting through a Walmart store and visiting two fast-food outlets before he was arrested.

The tragedy in the Miami suburb of Parkland, Florida, raised questions among anguished parents about the adequacy of school security measures and renewed a national debate on Capitol Hill and elsewhere about the epidemic of gun violence in American schools.

Thousands filled an outdoor amphitheater in Parkland Thursday night to attend a candlelight memorial vigil, where several speakers elicited thunderous applause from the crowd as they called for tougher gun control measures.

The Federal Bureau of Investigation disclosed it received a tip in September about an ominous online message that read: ‘I’m going to be a professional school shooter.’ The comment had been posted to someone else’s YouTube video by a person going by the name of Nikolas Cruz, now presumed to be the same individual as the suspect in the Florida shooting.

However, FBI agents had no information pointing to the ‘time, location or true identity’ of the person behind the message, Robert Lasky, special agent in charge of the FBI’s Jacksonville office, told reporters.

YouTube ultimately removed the material in question, and the FBI’s inquiry was dropped until the name Nikolas Cruz surfaced again in connection with Wednesday’s massacre at Marjory Stoneman Douglas High School.