An Antioch police task force engaged in racial discrimination because a disproportionate percentage of its investigations targeted African-American households in the federal Section 8 housing-assistance program, an expert witness for the American Civil Liberties Union argues in a report filed as part of an ongoing federal civil rights lawsuit.

In a response report, the city says the ACLU's research methods "violate standards of practice in social science research" and are not sufficient to prove discrimination.

The expert reports were submitted in September as part of the pretrial process in the class-action lawsuit filed in July 2008 by the ACLU of Northern California and three other Bay Area civil rights groups.

The ACLU's report was written by Barry Krisberg, president of the National Council on Crime and Delinquency. The city's primary expert was Brian Withrow, a criminal justice professor at Texas State University.

The city also commissioned reports by Vacaville police Chief Richard Word and former Los Angeles County sheriff's Lt. Joseph Callanan, who testified that Antioch police use standard policing methods.

The ACLU lawsuit argues that the Antioch Police Department's Community Action Team targeted African-American residents receiving Section 8 housing assistance in order to drive them out of Antioch.

According to the city, CAT is a response-driven unit formed to address residents' complaints of neighborhood

In the ACLU's expert report, Krisberg argues that the fact Section 8 rentals made up 4.7 percent of all Antioch households in 2006 but comprised 58 percent of locations CAT officers investigated shows police discriminated against Section 8 voucher holders.

He follows to say that "because African-Americans inhabit almost half of Section 8 houses, a CAT focus on Section 8 will necessarily disproportionately impact African-Americans."

According to Krisberg's analysis, African-Americans comprised 45.6 percent of Section 8 households in Antioch in 2006 but 66.7 percent of CAT Section 8 investigations. Furthermore, he says the Contra Costa housing authority's rejection of most voucher-termination recommendations for African-American residents referred by Antioch police suggests weaker evidence when referring them than other groups.

Between 2006 and 2009, 29.8 percent of African-Americans referred had their vouchers terminated, as opposed to 55.6 percent of whites and 52.8 percent of non-African-Americans overall.

In his response, Withrow argues that Krisberg fails to consider that being contacted by police is not a random event subject to mathematical probability — that other factors such as behavior come into play.

Withrow says CAT officers responded to citizen complaints without prior knowledge of the race or Section 8 status of the responsible parties, so they could not have intentionally targeted them.

"To the extent the plaintiffs' attorneys argue it's only about the numbers and demographics, then the rhetorical question is 'Why were less than 7 percent of Section 8 houses in Antioch ever CAT cases?' " said City Attorney Lynn Tracy Nerland.

This month the ACLU issued rebuttal reports.

"There is nothing in Dr. Withrow's report "... that would lead me to change any of the methods or conclusions in my original report," Krisberg wrote.

The lawsuit is continuing to move through the pretrial process, with a class-action certification hearing expected in early 2010.

Reach Hilary Costa at 925-779-7166. Follow her at Twitter.com/hilaryccosta.