(CNN) -- New Kids on the Block singer Jonathan Knight left fans confused and concerned Thursday when he walked off stage in the middle of a concert in New York.
According to reports, the five-member boy band was performing a mix of their old hits from the late '80s and early '90s, along with music from their new album, "10," when onlookers noticed that Knight seemed out of place.
HollywoodLife.com's Emily Longeretta was in the front row, and wrote on the celebrity website that Knight "seemed out of it. While the men started their choreographed dances, Jonathan stood uncomfortably in the background -- actually just standing and texting on his phone during the performance."
Billboard.com also reported that Knight seemed "visibly out of sync." He didn't join in when NKOTB got fans' arms waving with the memorable 1989 single "Hangin' Tough," and kept to the back of the stage as the other members worked to entertain the crowd.
The other members -- Knight's brother, Jordan, plus Danny Wood, Joey McIntyre and Donnie Wahlberg -- put their arms around him for support. Yet when Jordan said his brother was going to sing "Survive You" from "10," Knight made an attempt at the song, and then walked off the stage.
The rest of NKOTB went on singing as fans chanted for Knight's return, but he never came back. The singer has been vocal in the past about his struggle with anxiety, prompting speculation that that's what he was facing Thursday night.
On his Twitter account, Knight appeared to give fans a simple apology, tweeting "I'm sorry......" on Thursday evening. Earlier that day, he posted the hashtag "I feel naked," perhaps indicating his state.
NKOTB, one of the heartthrob boy bands of the late '80s and early '90s, came off a lengthy hiatus in 2008, and have issued two albums since then. They're also slated to go on a tour this summer with 98 Degrees and Boyz II Men, and Jonathan's performance Thursday had some questioning how many members of the group would make it.
But Wahlberg, for one, seemed unruffled.
"To recap," he tweeted after the New York event. "Joe was especially dazzling! Jordan was especially amazing! Danny was especially solid & Jon was especially shy! #Allsystemsnormal"