Mr. de Blasio, who has criticized the Board of Elections in the past for incorrectly purging voters from the rolls and for inefficiency, announced in August that the city would conduct “extensive, nonpartisan outreach to more than 561,000 inactive voters to ensure that they remain registered and to avoid any possibility of removing eligible voters from voter rolls.”

“The Board of Elections has not done a good enough job at communicating with voters,” Mr. de Blasio said at the time on NY1. “The City of New York is going to do that now more and more.” He added that the city had purchased a list of inactive voters that it was using to identify who to reach out to.

On Tuesday, the mayor’s office said that the list came from Civis Analytics, a business and political consultant. Such companies typically buy raw voter data from elections boards and sort and analyze it for their clients.

Efforts to reach Civis officials were unsuccessful. And the mayor’s office sought to blame the Board of Elections on Tuesday.

“It has come to our attention that a very small group of active voters may have received inaccurate letters from the city identifying them as inactive voters,” the mayor’s press secretary, Eric F. Phillips, said in a statement. “We’re working to get to the bottom of why the mailing list used, which originated with the city Board of Elections, seems to have led to this error.”

He acknowledged that people who got the letter might be “understandably confused” and encouraged them to check their registration status online. Raul Contreras, a mayoral spokesman, said that the letter was sent to more than 400,000 people.