Justin Guarini called it.

Last week, the former "American Idol" contestant said he thought the contestants "lip-synched" on group numbers.

Now a spokesman for the show's producer has confirmed that pre-recorded tracks were indeed being used.

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“Due to extensive choreography and to balance their voices with open mics against a screaming audience, the Idols do sing along to their own prerecorded vocal track during the group performances only," a rep for the show told "The New York Times," reiterating that all of the solo performances were 100 percent live.

Diane Macedo is a FOXNews.com editor who writes the "Reality Check" column and hosts the "Idol Chatter" video recaps every week for FOXNews.com, and has seen every "American Idol" episode of the current season. She says she has noticed occasional flubs in the group performances, but never thought the singers were using prerecorded tracks.

"Last week I saw Megan Corkery stop singing for one part as she tried to find her place next to Matt Giraud by the piano," Macedo said. "Once she got her footing she jumped back in. If something like that happened while someone was doing a solo performance or a duet, and what you were hearing didn’t reflect that, you would instantly know that they were lip-synching. But in a case like this, when it’s a big group singing together, it’s far less obvious."

Macedo, however, differentiates between what is happening on "Idol," and straight-up lip synching.

"They’re not going Milli Vanilli on us, they are singing to their own voices," says Macedo, refering to the band who won a Grammy for an album they did not actually sing on. "But if any of these contestants are incapable of organically singing harmonies, blending with other singers, or executing the countless other skills involved in group singing, that might influence my vote."

Fox, which airs "American Idol," had no comment.