Three-year-old twins died after being found unresponsive in a car parked along Interstate 95 in Miami early Friday and the female driver was critically injured when she jumped off an overpass when police arrived, authorities said.

The investigation had traffic tied up north of Downtown Miami and was delaying commuter trains.

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Miami-Dade County Police said 911 calls were received at about 2 a.m. about the boy and girl, who were in the backseat of a car parked on the Golden Glades Interchange where I-95 meets Florida's Turnpike. It is one of South Florida's busiest traffic spots. Passersby were attempting CPR.

When officers and fire-rescue arrived, the woman jumped off the overpass onto train tracks well below. Officers found the children, who were taken to the hospital and pronounced dead. No information on a possible cause has been released. The woman was also taken to the hospital, where police said she was in critical condition.

Police said they have not determined the relationship between the woman and the children. No names have been released.

"Right now our investigators are working to piece this tragic incident together," Detective Andre Martin told reporters.