An exterior brick wall of the post office was left blackened. The site of the crash was near several apartment complexes, restaurants and other businesses. Nearby residents said that their lights went out after they heard the crash.

In addition to Ms. McCord, the authorities identified the victims as Ian E. Biggs, 51, the pilot; Robert Vaughn Crisp II, 59; Gretchen D. Vincent, 51; and Michael Walker Vincent, 15.

The Ascension Episcopal School in Lafayette said the school community was mourning the deaths of Michael, a student at the school, and his mother, Ms. Vincent.

Only one of the six people aboard the two-engine Piper Cheyenne plane survived the crash. He was identified as Stephen Wade Berzas, 37, and was taken to a hospital in critical condition, fire officials said.

When it crashed, the plane struck a car in the post office parking lot, engulfing the car in flames and injuring someone inside, said Mr. Trahan, the spokesman. That person and two Postal Service workers were taken to local hospitals, he added.

Nearby hospitals confirmed receiving two people in critical condition. One was a woman on the ground who was taken to Lafayette General Medical Center and transferred to the University Medical Center New Orleans. The other person in critical condition was taken to Our Lady of Lourdes Regional Medical Center in Lafayette.

Information on the other two patients was not immediately available.

Lois Comeaux was at a Walmart near the post office when she heard a “big boom,” she said, and the store went dark. Ms. Comeaux, who lives about 300 feet from where the plane crashed, said that she could see pieces of the plane behind the post office, and that some of the post office’s windows had been blown out by the force of the crash.