U.S. immigration agents who conducted a recent operation in the Austin area arrested a higher percentage of people with no previous criminal convictions compared with other regions of the country and swept up more people found guilty of drunken driving than any other offense, federal documents obtained by the American-Statesman and KVUE-TV show.

The information, compiled by the U.S. Immigration and Customs Enforcement agency and presented late last week to several U.S. congressional officials, provides the most comprehensive data available in the national operation that sparked a furor in Austin, where 51 people were arrested.

It also intensified questions Wednesday about whom federal agents targeted as part of Operation Cross Check, which ICE officials have said was aimed at the most dangerous violent criminals.

The newly obtained records indicate a somewhat different outcome than federal officials had publicly announced.

Documents show that of the 51 people arrested by federal immigration agents, 23 were previously identified by ICE as having criminal convictions. Yet 28 were deemed "non-criminals" by the immigration agency — meaning those people did not have previous criminal convictions but were suspected of being in the country illegally.

The number of non-criminal people arrested during the operation number stands in sharp contrast to other regions where the operation took place.

According to the data, in the Austin region — which includes the cities of San Antonio, Del Rio, Laredo and Waco — 55 percent of those arrested were "non-criminal" immigrants suspected of being in the country illegally, compared with 34 percent in Atlanta, 30 percent in Chicago, 6 percent in Los Angeles and 5 percent in New York.

It was unclear why the Austin area had more of those non-criminal arrests, but federal officials said they suspect it could be the result of "collateral apprehensions" — the arrest of a person because they may be with a wanted individual at the time.

"During targeted enforcement operations, ICE officers frequently encounter additional suspects who may be in the United States in violation of federal immigration laws," ICE said in a recent statement. "Those persons will be evaluated on a case-by-case basis, and when appropriate, arrested by ICE."

ICE officials have declined to identify the 51 people detained as part of the operation during a four-day period during the second week of February, making it difficult to know why the immigrants were arrested and their previous criminal histories, if any.

A large number of the 51 people are thought to be from Austin, federal officials said.

The Statesman has been attempting to learn their names through other means, including family members and through limited federal court records.

Soon after the operation became public, ICE officials highlighted the arrests of a citizen of El Salvador who had pleaded guilty to the sexual assault of a child and of a Mexican citizen convicted of repeat domestic violence offenses.

According to the data, of the 23 who had been convicted, nine were convicted of drunken driving, two for assault and two for sexual offenses involving children. But the data include several individual arrestees who had been convicted of marijuana possession, obstructing the police and drug trafficking.

The operation sparked a panic in Austin, prompting protests and vocal responses by some city council members.