A yearlong investigation of Texas voter rolls has indicated that about 95,000 non-U.S. citizens might have improperly registered to vote, including about 58,000 who cast a ballot "in one or more Texas elections" since 1996, state officials announced Friday.

In an advisory to county voting officials, Texas Secretary of State David Whitley said his office used newly available data from the Texas Department of Public Safety to identify registered voters who might not be U.S. citizens.

The names were provided to the office of Texas Attorney General Ken Paxton, who promised to investigate and prosecute illegal voting activity.

"Nothing is more vital to preserving our Constitution than the integrity of our voting process, and my office will do everything within its abilities to solidify trust in every election in the state of Texas," Paxton said.

Casting a ballot when not eligible to vote is a second-degree felony that carries a prison term of two to 20 years.

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Almost 15.8 million Texans are registered to vote, about 79.4 percent of the voting-age population, according to the secretary of state's office

Travis County Tax Assessor-Collector Bruce Elfant, whose office handles voter registration, said he expects to get the list of suspected noncitizens Monday or Tuesday.

"We don't know how many are in Travis County. We haven't seen an individual list yet," he said.

Once the list arrives, county officials will mail a written notice telling suspected offenders: "Your registration status is being investigated because there is reason to believe you may not be a United States citizen."

Recipients will have 30 days to provide proof of citizenship — a birth certificate, passport or citizenship papers — or risk having their voter registration canceled.

"Nobody is going to get removed from rolls without the opportunity to show that they are citizens," Elfant said.

Elfant said he will be interested to see what those responses bring.

"This is a new data set. We don't know if it's a good data set ... or an erroneous data set. We’ll know a lot more in 30 days," he said. "The whole process of this is to determine whether that (list) was accurate or in error."

Beth Stevens with the Texas Civil Rights Project said she was concerned about the list's accuracy because state officials failed to fully disclose the methods used to compile names or to say whether naturalized citizens were properly accounted for.

"Notably, Texas has one of the largest numbers of naturalizations in the United States, with about 50,000 Texas residents becoming naturalized citizens each year," Stevens said. "There is no credible data that indicates illegal voting is happening in any significant numbers, and the secretary’s statement does not change that fact."

According to Whitley, his office matched the DPS data to the statewide voter registration database using documents that listed one of the following combinations:

• Last name, first name and full Social Security number.

• Last name, first name and the number on a DPS-issued driver's license, personal identification card or election identification certificate.

• Last name, first name, last four digits of the Social Security number and date of birth.

In the future, Whitley said, his office will use DPS citizenship information to cross-reference the statewide voter registration database once a month. If a noncitizen match is found, the county voter registrar will be notified to take action, he said.

Texas law requires registered voters to show one of seven forms of photo ID before casting a ballot: a Texas driver's license or handgun license, a U.S. passport, U.S. military ID card, U.S. citizenship certificate or a DPS-issued personal ID card or election identification certificate.

Voters without a photo ID can show documents that include their name and address, such as a utility bill or paycheck, and sign a "reasonable impediment declaration" stating that they could not obtain a photo ID because of a lack of transportation, disability, illness, work schedule, family responsibilities, lost or stolen ID or other reasons.

Update: The headline on this story was revised to more accurately reflect state officials' actions and findings.