MASSAPEQUA, N.Y.—When high school senior Natalie Ciappa nearly died of a heroin overdose Memorial Day weekend, she promised her parents she had learned her lesson and was going clean.

She got a job, met a guy and appeared to be getting better. She was even making her curfews again.

But on the first night of summer, Natalie went to a party and never came home.

Her parents called emergency rooms and the police, but they knew nothing about her daughter. They eventually learned where the party had been and headed there.

A woman answering the door led Doreen Ciappa to a side garage door. Natalie was motionless on a couch while MTV blared from a television. Red plastic cups were strewn across a catering table. Natalie’s lips and nose were blue: She was already dead.

“I knew it. I looked at her and I knew it,” she said. “I yelled, ‘Oh my God, someone call 911.”’

The death of Natalie Ciappa, a Plainedge High School honors student with a singing voice her mother says was too good for “American Idol,” has confirmed what police, prosecutors and federal narcotics agents say has been a growing problem on Long Island: Cheap, potent heroin available for sale in school hallways, malls, parks and just about anywhere young people congregate.

It is not a problem isolated to Long Island. While the federal Drug Enforcement Agency says heroin use has remained fairly consistent across the country in recent years, the highly addictive narcotic goes through vicious phases when it becomes the trendy drug of choice among teenagers. For example, suburban Dallas is among the areas combatting heroin use by kids as young as 8 for several years, officials said.

Fernando Cortez Sr. says his 15-year-old son and namesake died the first time he tried so-called “cheese,” a concoction of cheap black tar heroin mixed with over-the-counter medications that has killed dozens in the Dallas area. The potent mixture has been blamed in a wave of heroin deaths in recent years in other cities as well, from Chicago to Detroit to Philadelphia.

Cortez said Fernando Jr. was with his sister’s boyfriend in March 2007 when he was lured into trying the drug. “They did some, then they went and got more. This guy gave him way too much. He went to sleep and never woke up.”

Dave Cannata’s 16-year-old son, Nick, died in June 2005, with heroin and diphenhydramine in his system. He blames peer pressure.

“These kids are not going to get out from under peer pressure,” Cannata said. “I tell people if heroin is in your neighborhood, sell your house and get your freaking kid out of there right now. Get out of Dodge.”

On Long Island, the scourge of the drug is exacerbated by the fact that dealers are preying on areas with heroin that costs virtually nothing. One heroin ring that included Natalie Ciappa’s ex-boyfriend was recently charged with selling the drug at the Hempstead Bus Terminal for as little as $5 a packet.

“Cigarettes are $6 a pack!” exclaimed Patricia Silverman, whose children were all graduates of Plainedge High School. “My heart goes out to the parents. It’s just a national tragedy. This is supposed to be a beautiful community.”

Many teens using heroin these days snort the drug rather than inject it. “Unfortunately because it can be snorted, kids think the stigma of being a drug addict is removed,” said John Gilbride, the DEA’s special agent in charge of the New York office. “There’s not the same stigma as when a hard-core drug user injects it.”

Nassau County District Attorney Kathleen Rice says the paradigm of the heroin user has changed. “If you look at Natalie and you hear the description of her, you would never in a million years think that she was a child that would use heroin,” the prosecutor said. “You ask 10 out of 10 people what a typical heroin addict looks like and they’ll say it’s a junkie in the street with a needle hanging out of their arm.”

Because most kids are snorting, she said, “You’re not going to see the telltale signs like track marks and needle marks.”

Doreen Ciappa says she tried everything imaginable to get Natalie to stop using heroin.

For more than a year, there were battles at their suburban Long Island home: arguments over rehab, fights when she quit counseling, groundings after car accidents, threats about not going away to college.

In the beginning, Natalie admitted to smoking marijuana, but her parents were sure she was using something worse. She started losing weight and getting sick.

Doreen Ciappa started policing her MySpace page and searching her room for signs of drugs, finding the painkiller OxyContin among her belongings.

“I approached her and she told me she got it in school that day. I said, ‘In school?’ And she said, ‘Don’t you get it? At any time of the day in any hallway, if I want something all I have to do is look up and I will see somebody who can give it to me.”’

Then came the first overdose. The family was actually grateful because this was supposed to be their wakeup call. But three weeks later, Doreen and her husband Victor woke up on a Saturday morning and found no sign of Natalie.

“When your daughter has had one overdose and they don’t come home, we both immediately started crying,” Doreen said.

Doreen became frustrated with federal privacy laws that restrict the amount of health information she was able to get about Natalie after she turned 18 in March. Natalie was entitled to make her own decisions about rehab after turning 18.

At Natalie’s wake, Doreen Ciappa had a strange reaction when other parents greeted her with condolences, saying they seemed relieved that it wasn’t their child who was dead. Don’t assume it’s not your kid, she wanted to tell them.

“They look at their good, healthy, beautiful kid and they feel secure. I don’t want them to feel secure. And that’s what got to me at the wake, I saw these mothers.”