A 17-year-old North Carolina girl was charged with murder after she recorded and livestreamed a fatal shooting at a San Diego mosque this year, the authorities said on Monday.

The girl, Sarah Santiago of Forsyth County, N.C., was arrested last week, and a grand jury indicted her on three counts of murder by aiding and abetting and one count of conspiracy on Monday, according to Jim O’Neill, the district attorney.

Cain Clark, 17, and Caleb Vazquez, 18, killed three people at the Islamic Center of San Diego on May 18, an attack that was investigated as a hate crime and that heightened concerns about rising Islamophobia. Both were found dead inside a vehicle that contained anti-Islamic writings after the shooting.

The authorities have said that Ms. Santiago had published statements written by the attackers and recorded video of the shooting, which was filmed on GoPro cameras attached to helmets that the gunmen wore, Mr. Forsyth said.

Mr. O’Neill said in an interview that Ms. Santiago, who was enrolled in a public school in Forsyth County, knew the two gunmen who carried out the attack through their communications online. They never met in person, he said.

The trio would communicate on an online platform that served as “an echo chamber of hate,” the district attorney said. Their plan, he said, was to record their violent act and have Ms. Santiago disseminate it so that they would be “famous forever.”

After Mr. O’Neill and his staff met with federal law enforcement on Wednesday, Ms. Santiago was charged the following day with three counts of murder and one count of conspiracy.

“She ran with the pack — she shared in the kill,” Mr. O’Neill said, explaining the charges.

The shooting was the first ideologically motivated attack on a mosque in U.S. territory in a century, Mr. Forsyth said.

The victims were Amir Abdullah, Mansour Kaziha and Nadir Awad, who were all members of the mosque.

This is a developing story. Check back for updates.