The youngsters' deaths sent families into mourning on a day when other children were enjoying carefree play in huge piles of snow.
The fatalities prompted warnings from parents and officials about the dangers of a silent killer.
"My son was everything to me," said Torraine Thomas, the mother of Anthony Talbert, 12, a Baltimore youth who died in her car parked just around the corner from their house.
"Watch your kids in the cold and snow. If your car is warming up, crack the window."
City police identified the second boy who died with Anthony as Sheldon Mayers, 11. A third friend, Joshua Dyson, 11, survived after being sickened by the fumes.
Officials in Montgomery County said a 4-year-old girl, whose identity was withheld yesterday, died after inhaling car exhaust in a 1994 Honda that her aunt was digging out of a snowbank Monday afternoon.
And, in Carroll County, 12-year-old Glendon William Bell Jr. was found not breathing in a Ford Festiva that was running and almost entirely encased in snow.
Glendon, of the 500 block of E. Ridgeville Blvd. in Mount Airy, died later at Frederick County Memorial Hospital.
Police said the youths almost certainly died of carbon monoxide poisoning, although officials were awaiting test results to confirm causes of their deaths.
Last night in Northeast Baltimore, the father of Johnathan Thomas was curious about the whereabouts of his son. Police said the 20-year-old had left his house in the 1600 block of Northbourne Road late Monday to smoke marijuana in the car with Allen Adams, 17, of the 3500 block of E. Northern Parkway.
Police said Thomas' father went outside about 7 p.m. and discovered the lifeless pair in a car near an alley where Thomas usually parks.
It appeared they had been sitting in the car and were overcome by fumes from an exhaust pipe obstructed by snow, police said.
Carbon monoxide is an odorless, colorless gas emitted by motor vehicles and appliances that burn fossil fuels, including many furnaces and heaters.
It kills by depriving the bloodstream, and thus the body's organs, of oxygen.
In Mount Airy, police and grieving family members spent yesterday seeking explanations for Glendon's death.
The boy never regained consciousness after his father found him sitting in a running car behind a friend's house in the 200 block of Park Ave. about 3:45 p.m. Monday, police said.