The FBI has ended its investigation into the deadly Las Vegas shooting after finding no motive to explain why a shooter opened fire on a country music festival from a hotel room overlooking the concert grounds.

The end of the investigation comes nearly 16 months after gunman Stephen Paddock killed 58 people and injured some 900 others in October of 2017.

Paddock then took his own life from inside a hotel room on the 32nd floor of the Mandalay Bay Hotel and Resort.

Aaron Rouse, the special agent in charge of the FBI’s Las Vegas Office, said that Paddock had acted alone both in planning and carrying out the assault.

Why he did so, however, remains a mystery to investigators.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

The end of the FBI investigation follows after the Las Vegas Police Department ended its own probe in August. That department also failed to determine a motive in the case.

Paddock used 23 rifles and a revolver in his attack, and photos from inside the hotel suite where he launched the deadly attack showed something of a small arsenal left behind in the suite.

Another 18 firearms and thousands of rounds of ammunition were later found inside the gunman’s home in Mesquite, Nevada.

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An anonymous donor recently offered to donate $62,300 — the value of those weapons — in order to have the guns destroyed. The money would go towards financial relief for the families.