D.C. Police Chief Cathy L. Lanier, left, at a news conference Thursday, praised detectives on the case, as well as the Gun Recovery Unit. (Marvin Joseph/The Washington Post)

Police early Friday arrested a 17-year-old who they say threatened to shoot up his ex-girlfriend’s high school in Greenbelt using an AK-47 assault weapon that authorities said was found in his District home, along with 180 rounds of ammunition.

He had been sought since June 14, when the ex-girlfriend reported the threats she received via text messages on her cellphone, according to court records. The search warrant application filed by D.C. police says the teen threatened to come to Eleanor Roosevelt High School and “shoot it up” and “made a threat of mass violence when he told her that he was going to kill her along with anyone inside the school.”

Several parents of Roosevelt students said they were unaware of the alleged threat until they saw media reports Friday. The school system said the principal, Reginald McNeill, sent a letter home to parents three days after the suspect had threatened to carry out the attack.

Paul Dynan, who lives within walking distance of the school and will have a student there next year, said the notification came too late for parents to take precautions.

“It seems that lives are lost when this kind of information is sat on,” Dynan said. “I get more notices home about head lice than about a guy with an actual AK-47 and an ax to grind.”

D.C. Police arrested a 17-year-old on June 24 who they say was planning to shoot up Eleanor Roosevelt High School, his ex-girlfriend’s high school in Greenbelt, Md. (WUSA)

Prince George’s County school officials said two Greenbelt police officers and two county police officers were at the school on the final two days of classes — June 17 and June 20 — to provide additional security. They said that the principal sent a letter about the threat to parents June 20.

The letter advised parents that “an individual that does not attend the school said the threat would be carried out on June 17. Please be assured the threat was not carried out. Greenbelt City Police conducted an investigation.”

Roosevelt, with a magnet program in math, science and technology, is regarded by many as a top high school in Prince George’s County. It has 2,500 students, and while its seniors had already graduated before June 17, other students were in school for most of that week. June 17, a Friday, and the following Monday were early dismissal days.

The suspect, who is not being named because he is a juvenile, was being detained in the District on Friday awaiting extradition to Maryland, where court documents say he faces charges that include making threats of mass violence. He has not yet been charged in connection with the weapon. A D.C. police spokesman said detectives are still investigating links between the seized AK-47 and the young suspect.

Police also said that, in addition to the mass-violence charge, the teen has been charged in arrest warrants issued in Prince George’s County with possession of child pornography. Authorities alleged that he posted pictures of his ex-girlfriend in a sex act on public Internet forums because “she would not date him again.”

D.C. Police Chief Cathy L. Lanier, speaking Thursday, praised detectives on the case. “If anybody thinks for one second that the actions of the Gun Recovery Unit didn’t save countless lives, you’re making a mistake,” she said.

The search warrant application says the young woman’s mother contacted Greenbelt police on June 14. In one text message, according to police, the teen wrote to his ex-girlfriend “that he has guns and that she has seen his guns and knows he has them and will use them on her.” In another, police said he threatened to come to her house on June 17 “and beat the door down and kill her family.”

Greenbelt police contacted authorities in the District, who interviewed the young woman and obtained a search warrant for the young man’s apartment in the Kenilworth neighborhood of Northeast Washington. Police at the high school in Greenbelt also were notified.

The ex-girlfriend told police that she had been in that apartment in Kenilworth and had seen a handgun, kept under a couch cushion in the living room, that her then-boyfriend would carry it around the house “showing off.” The woman said that she saw a larger firearm in the teen’s bedroom.

The search warrant return filed in court says that police searched the apartment June 16 and found the assault rife, ammunition, spent shell casings ejected from weapons that had been fired, false identification cards, marijuana and a pellet gun.

Lynh Bui and Jennifer Jenkins contributed to this report.