Prosecutors who recently pursued convictions in the drug overdose death last year of a Woodbury High School student were reminded again just how essential the cellphone has become to teens negotiating drug deals.

“Every kid has a cellphone in their hands. They can communicate much easier, person to person, than they did in the past,” said Tony Zdroik, juvenile division chief in the Washington County attorney’s office.

Five criminal convictions this winter in the high-profile overdose death of Tara Fitzgerald, 17, came after investigators discovered more than 3,500 text messages documenting the sale of the fatal drug. Two of the defendants were 19 and three were 17 when the crime occurred.

Parents must be vigilant in monitoring their teenagers’ cellphones, Zdroik said.

“Too many kids who we see in real trouble are the ones whose parents walk the other way,” he said. When parents cease being the dominating influence in teenagers’ lives, drug dealers fill that void through contacts on social media, he said.

“I tell parents, ‘You need to continue to maintain the same message you told your child since they were 10 years old: Be good. Do what you’re supposed to. As soon as you stop telling that 15-year-old child who is using drugs that this is not good, then what other message are they hearing?’ ”

Parents “at their wit’s end” contact juvenile prosecutors seeking advice, Zdroik said.

When a teen who has excelled in academics and sports suddenly becomes defensive and prone to violent outbursts, parents might find an explanation in cellphone messages.

Parents might need to call police — not because they want to get their teenagers into trouble, but because they want their kids to know that what they did was wrong, Zdroik said.

“We get disappointed in juvenile court with how many kids who are achieving are involved with illegal substances,” he said, explaining the far reach of social media, including cyberbullying and distracted driving. “I doubt we’ll ever again have a drug distribution case that wasn’t arranged through text messages.”