Houston ? Immigration prosecutions shot up in southern Texas last year, far surpassing increases in any of the nation’s other 89 federal court districts, according to a study released Tuesday.

The report by the Transactional Records Access Clearinghouse at Syracuse University offers no explanation for the increase in immigration prosecutions in the Southern District of Texas, where the caseload grew from 4,062 in 2003 to 18,092 in 2004.

“We are doing more,” U.S. Atty. Chuck Rosenberg said. “Our folks on the border are working extraordinarily hard.”

But former U.S. Atty. Michael Shelby, who oversaw the Houston office from 2001 until June, says the south Texas numbers are misleading because of how the statistics were compiled. He said the Department of Justice did not include misdemeanors in its statistics of 2003 and 2002, when the program went into effect.

“And so it appears that all of the sudden between 2003 and 2004 there is this radical increase, when in fact, the increase happened two years before that.”

The study’s authors say nationwide immigration prosecutions, excluding the Southern District of Texas, show there were 19,868 prosecutions in 2004, compared with 17,149 in 2003. The study, however, indicates convictions dropped in 2004 to 14,690 from 15,376 a year earlier.