“I think the most important rule of thumb in introducing technology into voting is be extremely conservative,” he said.

Christopher C. Krebs, the director of the Homeland Security Department’s cybersecurity agency, said late Monday evening that the mobile app had not been vetted or evaluated by the agency.

The secrecy around the app this year came from the Iowa Democratic Party, which asked that even its name be withheld from the public. According to a person familiar with the app, its creators had repeatedly questioned the need to keep it secret, especially from the Iowa precincts where it would be used.

That person, who spoke on the condition of anonymity because he had agreed not to discuss details of the app, said that there were concerns that the app would malfunction in areas with poor connectivity, or because of high bandwidth use, such as when many people tried to use it at the same time.

“This app has never been used in any real election or tested at a statewide scale and it’s only been contemplated for use for two months now,” said David Jefferson, a computer scientist at Lawrence Livermore National Laboratory, who also serves on the board of Verified Voting, a nonpartisan election integrity organization.

J. Alex Halderman, a professor of computer science at the University of Michigan said, “This is an urgent reminder of why online voting is not ready for prime time.”

Mr. Jefferson warned that Nevada was also set to use a similar mobile app to report its caucus results in a few weeks.