PHILADELPHIA (CBS) — Three students at a West Philadelphia elementary school were hospitalized after eating baked goods that were possibly laced with an illegal substance. Police sources say the mother of one of the seventh graders purchased the baked goods.

The incident happened at the Martha Washington Elementary School on the 700 block of North 44th Street, shortly before 11 a.m. on Monday.

Philadelphia School District officials say the treats were brought into school by one of the students. Sources say it’s not clear if the student who brought in the baked goods knew it was possibly laced.

Police say a 13-year-old girl started feeling lethargic after eating the rice krispie treats and was transported to Children’s Hospital of Philadelphia in stable condition. Two other female students, ages 12 and 13, were also transported to the hospital in stable condition.

“They’ve been described as either breakfast treats or baked goods that was possibly laced with marijuana or an illegal substance,” said Philadelphia School District spokesman Lee Whack.

It is not known yet what the treats were potentially laced with.

Parents were startled to hear about what happened.

“It’s scary to know it was able to get into the school,” said Shaleena Pressley.

“As a parent, there’s always concern, but I’m also grateful for the school in how they took swift action,” said Charles Gilyard.

For its part, the school distributed letters alerting families of the situation.

“Once we found out about this incident, we notified School District of Philadelphia Police and were in communication with the paramedics and the hospital. A full investigation of this incident is ongoing,” the school said in a letter.

Police are investigating the incident.