From this:

After the review of data from Tarrant and Dallas Counties [homes of Dallas-Fort Worth, Texas], it appeared, at least on the surface, that some non-citizens were participating in U.S. elections.

Since 1976, 1,900 people have been removed from the voter rolls because of their citizenship status in Dallas County. Of those, 221 had voter histories.

Tarrant County election data from 2004 and 2005 shows 43 people have been removed from the voter rolls, but none had voter histories.

It goes on to present alternative explanations other than illegal aliens or legal visitors and immigrants voting, such as citizens not revealing their correct status in order to avoid jury duty. They also say that there's no check for citizenship status and the "only way to determine someone's citizenship is to see which box they checked on their voter registration card." State Representative Phil King tried to pass a bill requiring citizenship verification; Rep. Lon Burnam opposed it.