The town hall, which aired on NBC and MSNBC, was heavily criticized for the lack of challenging questions for the former vice president -- with critics calling the televised event a "Biden infomercial."
On Tuesday, a report by the Washington Free Beacon indicates that several of the participants were, in fact, not "undecided voters."
"NBC's town hall with Joe Biden & 'undecided voters' in Miami didn't seem to have undecided voters," Politico reporter Marc Caputo reacted.
Two of the attendees, one of whom moderator Lester Holt identified as a Hillary Clinton voter who had "voted Republican in the past," appeared on MSNBC back in August and were both identified by the network as Biden voters.
"If we get four more years of Trump, good luck. And good luck with the future attracting younger voters," Florida voter Peter Gonzalez said during an Aug. 21 program.
"Something that Joe Biden said during his speech that resonated with me was what defines America is possibility," Ismael Llano said in the same broadcast.
Another attendee, Matteo Gomez, previously told MSNBC on Sept. 30 that he would "probably" vote for Biden following the first presidential debate.
The Free Beacon also uncovered pro-Biden and anti-Trump social media posts from two other so-called "undecided voters" from the town hall.
NBC News and MSNBC did not immediately respond to Fox News' requests for comment.
Of the voters who were able to ask questions, one was described as "leaning" towards Biden, one was described as a "registered Republican" that was also "leaning" towards Biden, and one was described as a former registered Republican who had "voted for Hillary Clinton." Another questioner voted for Clinton in 2016 but according to Holt, he, too, voted for Republicans "in the past." None of the attendees were described as a Trump voter or "leaning" towards voting for the incumbent president.
Last month, ABC News faced similar scrutiny after the Free Beacon revealed that two of the "uncommitted" voters who attended a Trump town hall had criticized the president on social media.
Meanwhile, a Biden town hall hosted by CNN just days later identified 13 out of 16 participants as Democrats. The other three were identified as Republicans.