The teens were playing with illegal fireworks, when a mortar-style device exploded prematurely, according to investigators.
One teen may have lost a hand as a result and the others were also treated for injuries at a nearby hospital.
The incident happened around 7:30 p.m. Wednesday in a park near the corner of 5th Street and Avenue I.
Apparently the teens were gathered around a mortar-style firework, which either ignited prematurely in the tube or shortly after leaving the tube.
The incident prompted authorities to renew warnings for people not to play with fireworks - a caution that is typically not issued until the weeks leading up to July 4. Most fireworks, except those deemed "safe and sane" are illegal in Lancaster and many local communities.
"Unless it's a safe and sane firework, do not handle, do not attempt to ignite," said Craig Little with the Los Angeles County Fire Department. "According to the fire marshal, anything that explodes, flies up in the air or moves on the ground is an illegal firework - is not a safe and sane firework by definition."
No charges have been filed at this time against the juveniles and the incident remains under investigation.