Investigators believe a convicted murderer may have threatened to kill his child, prompting a lockdown at a Sandy Spring elementary school Thursday.

Ison Elementary School was locked down after a mother told police her child’s father threatened the two from his prison cell.

Anthony Tyrone Terrell was just 17 when he shot and killed his mother, a Gwinnett County sheriff's deputy, and two half-sisters, ages 4 and 11, in February 2008, authorities said. The crime spree was sparked by a fight between Terrell and his mother, Joy Deleston, who had forbidden her son to have a girl over to the family's Lawrenceville home without supervision. In 2010, Terrell pleaded guilty and was sentenced to two consecutive life sentences.

Now serving time in Wilcox State Prison, Terrell is accused of threatening his daughter and her mother through email, Channel 2 Action News reported. The child’s mother told police she received an email from her own account Thursday morning, but says it was sent by Terrell.

Police said it will be hard to prove that Terrell sent the threat, but took it seriously. The child’s mother called Ison Springs Elementary about the threat and Sandy Springs police placed the school on lockdown until the school day ended.