Actor Tahj Mowry, who has appeared in sitcoms “Full House” and “Baby Daddy,” wrote a heartfelt open letter to actress Naya Rivera, who has been missing since Wednesday.

The message, which he posted overnight Friday on Instagram, revealed private details of a relationship between the two.

“We grew up together. We became adults together. We experienced so many firsts together,” he wrote. “You were my first experience with everything: love, intimacy, heartbreak. We broke each others’ hearts and then mended them back together ... more than once. I will never not think of you.”

Mowry is most known for his role as T.J. Henderson in “Smart Guy,” a sitcom that aired from 1997 to 1999. He was just 10 years old when he started filming the project. Rivera appeared on the show twice as a child actress.

A romantic relationship between the two was never widely publicized, but in an interview with Vice earlier this year, Mowry said the pair dated multiple times.

“Our families were very close, so it was almost like an arranged marriage,” he said in the April interview. “She’ll say it was just a kid thing, but I think differently. There were three times we dated, and the last time was probably from age 20 to 23ish. Third time wasn’t the charm, but I wish her the best.”

Nivera also shared private information about their relationship in her 2016 memoir, “Sorry Not Sorry: Dream, Mistakes and Growing Up.” She noted that the two met at a photo shoot when they were 4, their moms became friends, and they later started attending the same church. She said she shared her first kiss with Mowry.

“Tahj and I continued to ‘date’ off and on for the entirety of our teenage years,” she wrote in her book, describing their relationship like that of best friends.

Mowry wrote that he is beyond “devastated” and called Rivera’s ongoing disappearance a “nightmare.”

“No woman has ever measured up [to] what you gave me or how you made me feel,” Mowry wrote. “I’ve never liked to admit it but I have never stopped loving you. A part of me always wished for the day where God would bring us back together to be what we dreamt we could have been.”

Rivera was declared missing earlier this week after her 4-year-old son, Josey, was found sleeping alone in a rented boat on Lake Piru, California, 55 miles northwest of Los Angeles. An adult life jacket and identification for Rivera were found in the boat. The boat had left the dock just three hours prior.

Josey told investigators that his mother had been in the lake without a life jacket and was unable to swim back to the boat.

Officials said Rivera’s car was found in the parking lot of the lake area and that she and her son had experience boating at the lake.

Thursday, officials said the 33-year-old was presumed dead and said rescue efforts had turned into recovery efforts.

Dozens of people, most of them divers, with help from helicopters, drones and all-terrain vehicles have been involved in the search on Lake Piru. Murky waters heavy with vegetation have made it difficult for divers to see more than about a foot ahead of them. In the area where the boat was found, the water was 30 feet (9 meters) deep, authorities said.

“In the lake, the visibility is terrible,” Sheriff’s Sgt. Kevin Donoghue said, suggesting that Rivera might have gotten caught in the same plants that have proved challenging for rescue divers. “If the body is entangled on something beneath the water, it may never come back up.”

Law enforcement officials believe Rivera may have drowned in a tragic accident.

Mowry said he still has hope.

“I know deep down you’ve always known how I felt. I look forward to the day where I can see your beautiful face once more and tell you everything I’ve wanted to say that I didn’t get the chance to say,” he wrote. “I love you forever. I always have and I always will.”

Mowry’s twin sisters, actresses Tia and Tamera Mowry, also posted messages on Instagram in honor of Rivera.

The Associated Press contributed to this story.

Cox Media Group