Four Philadelphia nuns were critically injured Sunday night when another vehicle crashed head-on into theirs on a Delaware County road, police said.

One of the nuns was flown to a Philadelphia hospital while the others were taken to Crozer-Chester Medical Center in Upland, Bethel Township Police Chief Tom Sharp said Monday.

The accident occurred about 7:10 p.m. when a vehicle driven by a 21-year-old Delaware County man, westbound on Route 322 near Foulk Road in Garnet Valley, entered the eastbound lane and collided with the nuns' vehicle. Sharp said it was not known why the man's vehicle went into the eastbound lane. The driver, who was alone, also was taken to Crozer-Chester in critical condition, he said.

The nuns are members of the Sisters of the Holy Family of Nazareth and live and minister in Philadelphia, a spokeswoman for the Des Plaines, Ill., religious group said Monday afternoon.

"To protect the sisters' privacy and to allow them undisturbed time to focus on healing, we are not yet releasing their names or information regarding their injuries," spokeswoman Tammy Townsend Kise said in an email. "We ask for continued prayers for our injured sisters and for the other driver involved in the accident," she said.

The Sisters of the Holy Family of Nazareth serve families through ministry in schools, hospitals, parishes and social-service agencies in nine states and 13 other countries.

Due to the severity of the injuries, the Delaware County District Attorney's Office is investigating, police said.