Marked increase in Internet use. Online chat shifting to a cell phone. A normally open child being quiet and secretive.

If parents see this behavior, they should sit up and take notice, as their child might be falling victim to an online predator, an FBI expert on cyber criminals said on Monday.

“It’s safe to say that there are people out there right now targeting children in our area,” said FBI Special Agent Scott Durivage.

His words came after a then-Abington teacher and coach targeted students last year on Facebook, was fired, and is now serving three years of probation after pleading guilty in Brockton District Court to three counts of sexual conduct for a fee.

The case against Jon J. O’Keefe began when the victims came forward and reported his inappropriate behavior.

Durivage said it is important for victims to “find someone they trust, and let them know time is of the essence because forensic evidence may not be out there long.”

O’Keefe, who was serving at the time as the boys’ tennis coach and substitute history teacher, was fired in May when students came forward after being propositioned by him.

The 31-year-old Waltham resident offered to buy students alcohol, said he would pay them for sex, and agreed to write a letter of recommendation in return for sexual favors, according to a police report filed in the court.

O’Keefe contacted the victims through Facebook and text messages, telling them to delete any messages from him after their conversations, authorities said. He originally claimed his Facebook account had been hacked.