LAC CRUCES, N.M. (KFOX) — All four of the Young Park shooting suspects pleaded not guilty Monday morning after three people were killed and 15 injured in March during a mass shooting at a New Mexico car show.

Doña Ana County District Attorney Fernando Macias said all four suspects are being tried as adults.

"All four are going to be joined together, at least initially in terms of their cases. And again, we anticipate going forward, all four of them are being tried as adults," Macias said.

The four suspects are 20-year-old Tomas Rivas, 17-year-old Nathan Rivas, 17-year-old Gustavo Dominguez and 15-year-old Josiah Ontiveros.

The defendants have been formally indicted on charges of first-degree murder, including conspiracy to commit murder.

Two 19-year-old men and one 16-year-old boy were killed, with the injured ranging in age from 16 years to 36 years.

Officials said all four suspects have a criminal history. Three of them had a criminal history together.