PORTLAND, Maine — The parents of 6-year-old Etan Patz — who disappeared 33 years ago Friday after leaving his Manhattan home heading for the school bus — got the news this week that they have been dreading for decades.Their son is believed dead and a 51-year-old man has admitted to strangling the first-grader in 1979 when he himself was a teenager.

“The pain of losing a child never dulls,” U.S. Attorney Thomas E. Delahanty II said Friday in Portland as he and other law enforcement officials marked National Missing Children’s Day. “For those thousands of families missing children today, like Etan Patz, whose case lingered unsolved for … years, we don’t give up.”

In 1983 President Ronald Reagan declared May 25 — the day Etan Patz vanished four years earlier — as National Missing Children’s Day, and the following year Congress passed the Missing Children’s Assistance Act, creating the National Center for Missing & Exploited Children.

Toddler Alya Reynolds, who was 20 months old when she was reported missing on Dec. 17 from her father’s Waterville home, is one case that police continue to actively investigate, but she is not the only missing child in Maine.

“In Maine, there are currently six unsolved missing children cases dating back 40 years,” Delahanty said. “They are not all infants or toddlers.”

In addition to the Ayla Reynolds case, Douglas Charles Chapman, then 3, of Alfred was reported missing June 2, 1971; Cathy Marie Moulton, 16, of Portland was reported missing Sept. 24, 1971; Kurt Ronald Newton, 4, of Manchester was reported missing Sept. 1, 1975; Bernard Ross, 18, of Ashland was reported missing May 12, 1977; and Kimberly Ann Moreau, 17, of Jay was reported missing May 11, 1986.

Chapman was last seen playing by a sandpile about 25 yards from his home in Alfred, while his mother was inside on the phone and his father was at work, according to a Maine State Police website dedicated to missing Mainers.

Moulton had dyed red hair and was last seen in downtown Portland, according to the National Center for Missing & Exploited Children’s website.

Newton wandered away from his family’s campsite at the Chain of Ponds Public Reserve Land near Coburn Gore on the Quebec border. He was last seen riding his tricycle at the campsite while his mother was out of sight washing muddy shoes.

Moreau was last seen in the company of an individual she met earlier in the day and foul play is suspected, the state police website states.

Two other teenagers who disappeared years ago also remain unaccounted for.

Bonnie Ledford, 19, of Dedham, who went missing in 1980, and Angel Antonio Torres, also 19, of the Saco-Biddeford area who was reported missing by his family on May 24, 1999 are listed on the state police website.

Foul play is suspected in both cases.

When children go missing or are abducted, time is of the essence, said Todd DiFede, the FBI’s senior supervisory agent for Maine.

“Every second, every minute and every hour counts in bringing a child home safely,” the veteran agent said.

To help parents keep vital information at their fingertips, there is a new app for smartphones that records a child’s height, weight, eye color and physical traits, as well as a photo, and can be instantly accessed, if needed.

“With the click of a button, the information is sent in an email,” DiFede said.

The National Center for Missing & Exploited Children also has several tips for parents when they discover their child missing.

The new smartphone app is “a tool all parents and grandparents should be aware of and make use of,” Delahanty said.

Parents should always be aware of where their kids and teens are and should know that criminals who take children come in all shapes and sizes, said Maine State Police Lt. Brian McDonough, director of the Major Crimes Unit.

“Predators are everywhere and they come from all walks of life,” said the lieutenant, who is the liaison to the National Center for Missing Children and Maine’s AMBER Alert coordinator.

The disappearance and 1932 murder of 20-month-old Charles Lindbergh Jr., son of the world-famous airplane pilot, drew worldwide attention and led to the Lindbergh Law, which allowed law enforcement to pursue kidnappers across state lines.

When little Etan Patz went missing in 1979, the media frenzy again put a national spotlight on abducted children. He was the first missing child to ever appear on a milk carton, a tool that is now commonplace, and the Missing Children’s Assistance Act led to the creation of the AMBER Alert, an early warning system issued by law enforcement to notify broadcasters and state transportation officials when children are abducted.

“There is no rest for a parent who has lost a child, and there should be no rest for any of us who are in a position to help,” Delahanty said, flanked by DiFede, McDonough, Deputy U.S. Marshal Mike Tenuta, South Portland Police Chief Ed Googins, Oxford County Sheriff Wayne Gallant, victim witness advocate Heather Putnam and Assistant U.S. Attorney Stacey Neumann.

Delahanty said education for children goes a long way toward helping them protect themselves.

“We ask parents and guardians to take just 25 minutes to teach their children some safety tips that may save their lives someday,” he said, adding that educational tools are available online at the National Center for Missing & Exploited Children’s Take 25 campaign website, www.take25.org. “Twenty-five minutes for 25 tips.”

The tips include telling children never to accept rides from anyone unless they have parental permission, always walking with a friend or in a group, and knowing how to contact loved ones at home and work.