PITTSBURGH -- Nearly 15 years after a house fire killed a 3-year-old girl and a 2-year-old boy, their mother and a friend have been arrested for allegedly starting the blaze.

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Video: Suspects Arrested; Watch Marcie Cipriani's Report

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Tequilla Fields, 34, the mother of Charita and Montelle Thornhill, faces homicide charges, along with Lachan Russell, 29, who proclaimed her innocence and blamed Fields as police took her to the Allegheny County Jail on Wednesday morning.

The Thornhill children died when their grandmother's Apple Street home in Pittsburgh's Lincoln-Lemington section went up in flames on July 11, 1990. Police determined that someone set the family dog on fire, which caused the house to burn.

Fields, in a 1990 interview with Channel 4 Action News, said she came home late and noticed that the dog smelled of kerosene. She claimed that she went out again to a store and returned to find the house on fire.

Detectives originally questioned Fields, then 18 and named Tequilla Newsome. No charges were filed at that time.

Pittsburgh Police Deputy Chief William Mullen, who worked the case as a lieutenant years ago, told Channel 4 Action News that he was never able to forget about the two deaths.

"These children were innocent and killed by a senseless act," he said Wednesday.

Mullen suspected Fields was lying, so he handed the investigation over to a cold case unit, which was able to crack the killings.

The new detectives on the case drew motivation from pictures of the young victims' faces.

"It was almost like they (the children) were saying, 'Don't nobody care about us,'" Cmdr. Maurita Bryant said Wednesday. "They put the pictures on their desks and they just started working the case."

Investigators now say the suspects tied the dog to the porch and set the animal on fire because Fields' grandmother wouldn't get rid of it, even though Montelle Thornhill was allergic.

More charges, including arson and conspiracy, are pending against Fields and Russell, police said.