A Mandeville teenager was found dead at Columbia Street Landing in Covington on Sunday night after apparently using illegal drugs, St. Tammany Parish Coroner Dr. Charles Preston said Monday.

Alexander James Shelby, a senior at St. Paul’s School, and a friend had apparently been using drugs when both blacked out, according to the Coroner’s Office. When the friend awoke, Shelby was face-down in a flooded ditch.

An autopsy is pending, and an official cause of death has not been determined. But investigators believe the teenagers had been smoking Mojo, a synthetic marijuana.

“This tragedy only underscores the dangers associated with drug use, particularly the synthetics that have infiltrated our community in recent years,” Preston said.

He encouraged parents to initiate discussions with their children about the risks of using drugs.

Brother Ray Bulliard, president of the school, sent a letter to parents urging them to talk to their sons about their feelings about the loss and the reason for his death.

He said the school “strongly endorses” the coroner’s statement.

Follow Sara Pagones on Twitter, @spagonesadvocat.