Two adopted teenagers have been arrested in Spain on suspicion of killing their mother after she was found dead in a car inside the family’s garage with a bag over her head and a "stab wound to the neck," a report says. 

The juveniles, who were adopted from an Eastern European country and are 15 and 13 years old, were tracked down by Spanish authorities in Castro Urdiales early Thursday morning, hours after police discovered the remains of Silvia López Gayubas, according to the El Diario Montañés newspaper. 

The outlet reported that the juveniles first called their grandmother, who lives in an adjacent property in Castro Urdiales, on Wednesday night to tell her they had been kidnapped. She then alerted police. 

When police arrived at the family’s home to investigate, they discovered the 48-year-old mother in the "back seat of a car with a bag over the head, with blows and a stab wound to the neck," El Diario Montañés reported. 

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Authorities then launched a search for the juveniles who were missing from the home. 

When investigators called their father, who was working the night shift at a metallurgical company in the region, "he didn't believe it, he was devastated," a source told the newspaper. 

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The boys were eventually located in Cotolino Park in the coastal Spanish town, and one of them tried to run away from police before being taken into custody, according to El Diario Montañés, which added that they did not confess to the killing. 

A regional government official then told the newspaper Thursday that the juveniles were being held on suspicion of killing their mother. 

"Our feeling is one of absolute sadness. It is regrettable that acts like this occur," Castro Urdiales Mayor Susana Herrán told El Diario Montañés. 

The motive for the killing is unclear.