Authorities said they are also questioning two teenagers about the case.
Authorities said Nancy Wood and her 3-year-old son, Robbie, died in the hit-and-run accident early Saturday morning on Highway 39 in Johnston County. Funeral services were held Monday.
Authorities had been looking at the broken plastic pieces found at the scene to find clues about what type of vehicle may have struck them.
The van was taken to the State Bureau of Investigation's crime lab for inspection.
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