BOSTON -- The off-duty security officer who shot and killed a psychiatric patient Tuesday after the patient stabbed a Massachusetts General Hospital doctor did "what anyone else would do," and said he doesn't consider himself a hero.

Paul M. Langone, 33, was inside the Boston office building when he heard screams and found 37-year-old Jay Carciero stabbing Dr. Astrid Desrosiers. Langone shot and killed Carciero when he refused to drop the knife, police said.

Langone briefly spoke about the incident with reporters outside his parents' Reading home Friday morning.

"I definitely don't consider myself a hero. I am OK," Langone said. "I just did what anyone would have done."

He said his thoughts and prayers were with the Carciero family.

"I'm OK, just trying to get back to a normal life -- go to work, take my dog out, and go about my normal business," Langone said.

Desrosiers' husband, Claude, called Langone's father Friday and thanked him for his son's courage.

"He said if my son got there any later, he feels his wife would not be alive," Langone's father, Paul F. Langone, said.

Although the investigation is ongoing, Suffolk County District Attorney Dan Conley said Thursday that Langone, who is licensed to carry a weapon, acted in self-defense.

"The evidence at the scene, Mr. Langone's statements to the police and accounts of multiple witnesses all indicate that Mr. Langone acted lawfully when he discharged his firearm. There is strong evidence that he saved the life of Dr. Desrosiers and perhaps several other people," Conley said.

Carciero's family members said they are at a loss to explain the events.

The family released a statement on Thursday, saying that he was a gentle and caring person and a devoted and loving father to his four children.

"We never would have imagined that the events that took place at the doctor's office on Tuesday were even possible. What happened that afternoon remains unexplainable to us because Jay was known to his family and friends as such a gentle and caring person," the statement said.

The family expressed its thoughts and prayers for the recovery of Desrosiers, who is in serious but stable condition.

A trust fund named the "Carciero's Children Fund" has been set up in the name of Carciero's children at the Stoneham Municipal Federal Credit Union.