UnitedHealth suspect Luigi Mangione has been indicted on new charges , including a first-degree murder offense that prosecutors said was committed to further an act of terrorism.

“This was a frightening, well-planned, targeted murder that was intended to cause shock and attention and intimidation,” Manhattan District Attorney Alvin Bragg said at a news conference Tuesday.

The Dec. 4 slaying of UnitedHealthcare Chief Executive Brian Thompson shocked the country and launched a dayslong manhunt for the shooter. Police say Mangione targeted Thompson, waiting an hour outside a Hilton hotel in Midtown Manhattan and shooting the 50-year-old in the back and leg while the CEO was on his way to UnitedHealth’s investor day. Mangione’s attorney declined to comment.

Mangione was indicted on a count of murder in the first degree, murder in the second degree, and murder in the second degree as a crime of terrorism, among other charges, according to an indictment unveiled Tuesday. In reference to the terrorism allegations, the indictment described the killing as a violent act that was intended to intimidate or coerce.

Mangione was arrested in a McDonald’s in Pennsylvania earlier this month after the 26-year-old was seen eating in the restaurant . Officers said he was in possession of a ghost gun, 3D-printed silencer and handwritten notes that criticized the U.S. healthcare system. He is currently being held in custody in Pennsylvania , where he faces firearm and forgery charges.

Mangione has a court hearing in Pennsylvania on Thursday, which could pave the way for him to come to New York to face the murder charges. He faces a maximum penalty of life in prison without parole for the New York charges, Bragg said.

Since Mangione’s arrest there has been intense public interest in when his parents, part of a large and prominent Baltimore family , came to realize that their son was a suspect in a brazen killing that had captivated the nation . The family had been desperately searching for him for the better part of the year.

People close to the family said he had cut off contact. One said that he “went off the grid six months to a year ago and wasn’t communicating with anybody,” and that his distraught mother was doing all she could to find him. Another said the Ivy League engineering graduate was “MIA for about eight months.”

In November, his mother, Kathleen Mangione , reportedly contacted the San Francisco Police Department to report her son missing.

Joseph Kenny , New York Police Department chief of detectives, said an officer in San Francisco working on Mangione’s missing-person case reached out to authorities after seeing a photograph distributed by New York police, citing a resemblance.

Police spoke to Mangione’s mother the weekend before he was arrested, Kenny said.

“She didn’t indicate that it was her son in the photograph,” he said, “but she said it might be something that she could see him doing.”

Kenny said the information from Mangione’s mother was going to be passed along to detectives, but the Pennsylvania police apprehended him early Dec. 9 in Altoona, Pa., before they could act on it. On the day before, Luigi Mangione’s parents had attended a church event in Baltimore’s Little Italy, honoring one of Kathleen Mangione’s brothers. “Less than 24 hours later, their lives completely did a flip-flop,” said Santo Grasso , a longtime family friend who chatted with the couple there.

The family released a statement the day of Mangione’s arrest, saying “we are devastated by this news” and offering prayers to Thompson’s family.

Thompson’s killing tapped into anger against health-insurance companies and their executives. Some posts on social media seemed to revel in Thompson’s death, while others held up Mangione as a folk hero.

At the news conference, NYPD Commissioner Jessica Tisch admonished social-media posts that celebrated Thompson’s killing and criticized posters hung in New York City that targeted other chief executives.

“We don’t celebrate murders, and we don’t lionize the killing of anyone,” she said. “And any attempt to rationalize this is vile, reckless and offensive to our deeply held principles of justice.”

Corrections & Amplifications undefined Brian Thompson was chief executive of UnitedHealthcare. An earlier version of this article incorrectly said he was CEO of UnitedHealth. (Corrected on Dec. 17)

Write to Joseph Pisani at joseph.pisani@wsj.com and Scott Calvert at scott.calvert@wsj.com