Exactly 25 years after a 13-year-old girl was violently attacked and murdered in her family's backyard, the Ontario Provincial Police are appealing to the public for information in the cold case.

On Sept. 1, 1990, Leah Sousa was asleep on the couch in her family's home in Cumberland Beach, Ont., when an intruder broke in through a back door shortly after midnight. Cumberland Beach is located northeast of Orillia, Ont.

According to investigators, Sousa's mother, Lora, was attacked with an unknown object and was later found unconscious on the living room floor. OPP says Sousa was sexually assaulted, before she was taken into the backyard and killed. Lora Sousa's son, then nine months old, was found unharmed in his crib.

Speaking on the 25th anniversary of the teen's death, Lora Sousa said her daughter was "robbed" of her life.

"I'm really upset; 25 years is a lifetime. My daughter would be 38 years old now," Lora Sousa told CTV Barrie on Tuesday. "I feel utterly helpless. I cannot stand it."

Police say Leah's body was found the following day in the backyard. She died of severe head injuries.

The murder weapon, which police believe was a metallic object, has never been found.

Investigators who have been assigned to the case over the years have reviewed a file that is believed to be one of the largest in OPP investigative history. Sgt. Peter Leon said thousands of tips that have been received, even as recently as the past several days. Since the investigation began, officers have followed up with more than 1,500 interviews.

"Each and every one of those tips that have been received has been followed up and thoroughly investigated," Leon said.

Investigators are hoping a fresh appeal for witnesses to come forward will help them solve the murder.

"I strongly believe that there is someone who still resides in the area possesses the information that will help us solve this crime," Det.-Insp. Dave Robinson said in the statement.

Leon said one of the OPP's "biggest" concerns at this point is that any potential witness may be older now.

"(There's) a potential that a person out there has information and perhaps is elderly and perhaps may leave this world with that information."

A $50,000 reward remains in place for any information that leads to the killer's arrest and conviction.

Anyone with information is asked to contact OPP at 1-888-310-1122 or Crime Stoppers at 1-800-222-TIPS (8477). A special tip hotline has also been set up at 705-329-7588.

With a report from CTV Barrie's Mike Walker