Langone declined to speak about the continuing investigation, but said he was “very pleased” with the district attorney’s preliminary report that the shooting was justifiable and he was “just trying to get back to normal life.”

He also said he planned to continue to pursue his career goal of becoming a police officer.

Paul Langone Sr., the security guard’s father, spoke to Claude Desrosiers, husband of the injured doctor, Astrid Desrosiers, later in front of the Reading home after being handed a cellphone by a reporter.

The father could be heard saying, “Thank you” a number of times to the husband. “Thank you very much for your kind comments. We are praying for your wife’s complete recovery,” he said.

“He’s very appreciative … of what my son did. He feels his wife might not be alive” if the young man had not acted, the father said.

The father said that Claude Desrosiers had also told him that his wife, who remains hospitalized, “needs prayers.”

Suffolk District Attorney Daniel F. Conley said Thursday that, based on a preliminary investigation, Langone may have saved the life of Dr. Desrosiers and others and the shooting appeared legally justified.

Desrosiers was pinned to the floor and being stabbed by Carciero at about 2 p.m. Tuesday in the hospital’s bipolar clinic on the fifth floor of a building at 50 Staniford St. As clinic colleagues fled, another doctor tried to pull the 250-pound Carciero off her, but fell back when the patient slashed at him, authorities said.

Langone, an off-duty security officer carrying a concealed handgun, may have been her only chance, authorities said. He entered the room, ordered Carciero to stop, and when he didn’t, fired multiple shots, hitting Carciero twice.

Speaking at a press conference Thursday in Boston, Conley said all the evidence, including witness accounts and Langone’s statements to police, indicated that Langone acted lawfully in killing Carciero, 37, a married father of four young children who had long struggled with mental illness before turning a knife on his therapist.

Authorities’ findings undercut accusations by Carciero’s attorney that Langone had acted rashly in using deadly force. Langone, a 33-year-old former boxing standout who has worked as a doorman at Boston bars, has been widely hailed for his unhesitating bravery in rescuing Desrosiers, a 49-year-old psychiatrist from Belmont.

Desrosiers has not been interviewed by police yet because of her injuries. She is expected to survive, but remains in serious condition.

When police arrived, Langone identified himself as the shooter, surrendered his gun, and submitted to a lengthy interview by investigators. He was then released from custody and has not been charged with any crime.

Because Carciero and Langone live just 2 miles apart in Reading, investigators explored potential connections between the men, but found none, Conley said. Family members had previously said the men did not know each other.

Langone was at a scheduled appointment on the fifth floor of the high-rise office building when the attack occurred, Conley said. Besides the Bipolar Clinic and Research Program, the floor contains several other medical and research offices, Conley said, declining to be more specific.

Conley emphasized that the investigation is in its preliminary stages and will remain active until a final decision on criminal charges is made.

Langone was authorized to carry a weapon in his capacity as a sworn special police officer, a title given to security personnel who work for private firms but are licensed by Boston police.

As she left her Reading home Thursday for a midday Mass, Langone’s mother said her son “doesn’t feel like a hero.’’

“Anyone else would do the same thing under the same circumstances,’’ she said, declining to give reporters her name. “We’re very proud of him. He’s a remarkable young man, and he always has been.’’

“There are two Reading families in a very unique and delicate situation,’’ she continued. “Out of respect for the other family, I’m not going to say anything.’’

Carciero’s family issued a statement Thursday saying that he was a gentle and caring person and that what occurred Tuesday “remains unexplainable to us,’’ the Associated Press reported.

The family expressed its thoughts and prayers for the recovery of Desrosiers and said it was establishing a trust fund for Carciero’s children through the Stoneham Municipal Federal Credit Union, according to AP.

Brian Ballou of the Globe staff contributed to this report.