They were singing Christmas carols at a church to raise money for the homeless, and while as many as 500 spectators listened to the chamber choral ensemble, someone sneaked into a basement room and rifled through the performers’ belongings.

Before the We All Nowell concert had ended Sunday evening at St. Paul’s Lutheran Church in Northwest Washington, charges had been rung up at retail spots across the region — $600 at an REI and $150 at a Toys R Us on one card taken from a performer.

The leader of the Thomas Circle Singers said 15 debit or credit cards were taken from a half-dozen members, and that thousands of dollars in purchases were made before anyone discovered the cards missing. He said one person also lost a personal identification card.

D.C. police said Thursday that they are investigating as victims continue to call and file complaints. The thefts occurred between 4 and 6 p.m. at the church, in the 4900 block of Connecticut Avenue NW.

“I think we’ll be more careful, but I feel like this person was in need as well,” said Juliene James, who has been with the group for eight years and works in criminal justice. “I feel grateful that it wasn’t more damaging, but of course there’s always the feeling of violation and sadness when things like this happen. We’re going to continue with our mission and our concerts.”

The 41-member Thomas Circle Singers group was formed in 1976 to help raise money for the homeless. Sunday’s concert was to aid a group called Thrive D.C., which helps people who need shelter.

The choir members had left their items in a room in the church basement where they had prepared before the concert. The room was not locked during the concert.

“We all warmed up and left our things,” said Clark Cheney, the group’s chairman of the board. “We got lined up and began our processional. Literally as we were singing, underneath our feet someone was rifling through wallets, purses and bags.”

“While we were trying to help people in need we were being robbed,” Cheney said. “It was terribly disappointing. . . . We didn’t even think someone would want to do that kind of harm.”

Six people in the choir had items stolen, Cheney said. He said the choir felt disheartened about the theft, but that the singing performance was good. He estimated that the singers raised several thousand dollars for Thrive. One member didn’t notice her missing credit cards until she was at dinner later with friends.

“People had to shut down their credit and debit cards right in the middle of the Christmas season,” he said. “It was just incredibly disruptive.”