Castellano was hit by a car and killed Sunday morning on Interstate 195 while searching for a second person from an abandoned a car.
The Monmouth County Prosecutor's Office has not charged the driver who struck Castellano. But the woman who allegedly abandoned the car, Diana Hoffman, is being held on several charges, including creating a false public alarm for misleading authorities about being with another person.
The investigation is continuing.
Castellano's best friend in the state police, fellow firearms instructor Richie Hallihan, remembered Castellano as a perfectionist who was deeply dedicated to his family and career. A high school football star and fitness fanatic, Castellano last fall hung a 2-inch thick rope from a 30-foot tree in his yard, which he would climb to build upper-body strength.
"Marc was a great, great, great guy who would do anything for you," said Hallihan.
Castellano never served in the military, but Hallihan said he possessed the discipline and self-pride that would have made him a good Marine.
After helping Hallihan move to a new home in Jackson, Castellano liked the neighborhood so much that he bought a lot nearby. The foundation had been poured on the new house just before Castellano was killed. The high school sweetheart who is now his widow has decided not to move.
Sgt. Thomas Rasmussen, who supervises the unit Castellano was assigned to, called the fallen officer "a model trooper."
Colleagues remembered Castellano's eagerness, his positive spirit and dedication to the state police.
He took a day off from work last month to receive his master's degree from Fairleigh Dickinson University - the advanced degree needed for him to advance his state police career.
Castellano was the first trooper since 2004 to die in the line of duty. Col. Rick Fuentes, superintendent of the state police, said it's always difficult to lose an officer.
"We're like a large, large family," Fuentes said of the 3,005 enlisted members and 1,327 civilians who make up the state police. He said he's always been struck by "how the organization becomes so personal so quickly when something happens around the family."
There has been a steady flow of troopers to the Castellano home since Sunday. And within days, colleagues had established a fund through TD Bank for his children, ages 1 and 4.
Gov. Chris Christie ordered flags flown at half-staff all week to honor the Howell resident. A viewing will be held Friday, with the funeral planned for Saturday.