An American motorist was arrested with the bodies of her young nephew and niece in the trunk of her car, police said Friday, after making the macabre discovery during a routine traffic stop.

Nicole Johnson, from the east coast city of Baltimore, faces multiple charges including child abuse resulting in the deaths of the seven-year-old girl and five-year-old boy.

The 33-year-old had stuffed her niece's body into a suitcase and put it in the trunk as long ago as May last year, continuing to use the car normally, according to the Baltimore Sun.

She placed the boy's body next to his decomposed sister a year later, wrapped in a plastic bag, the paper reported.

Police stopped her for speeding on Wednesday and decided to impound the car when it emerged she did not have the correct papers.

According to the paper, one officer told Johnson the vehicle was going to be towed and she replied: "It don't matter, I won't be here in five days."

She added that "y'all going to see me on the news making my big debut," the paper said, citing a police report.

The two children had been entrusted to Johnson's care, she said, in 2019 by her sister.

Johnson admitted during questioning that she had hit her niece several times, the Sun said, causing the youngster to hit her head on the floor. She did not explain how the boy died.