The promised string of legal challenges to how law-enforcement agencies are applying Arizona’s immigration law, SB 1070, is under way.

On Friday, the American Civil Liberties Union filed its second legal challenge, alleging civil-rights violations.

The claim against the Tucson Police Department alleges that officers engaged in racial profiling and illegally detained two immigrants during an Oct. 8 traffic stop. The incident drew dozens of protesters in an impromptu effort to stop Border Patrol agents from taking the driver and passenger.

Tucson City Attorney Mike Rankin said the city will evaluate the claim like any other it receives. He wouldn’t comment further on pending legal action.

A claim is the first step in suing a government. The ACLU and the city of South Tucson are in negotiations after the first claim was filed against that city in November.

A provision of SB 1070 that took effect in September 2012 requires local law enforcement to try to check the immigration status of anyone they stop if they come to believe those suspects are in the country illegally.

Although it blocked several provisions, the U.S. Supreme Court allowed the so-called “show me your papers” part to take effect because it found a “basic uncertainty” about what it actually requires of law enforcement officers.

Since then, the ACLU has been monitoring local agencies’ implementation because of worries that the law unconstitutionally allows illegal practices. The organization has received hundreds of calls from across the state, said Christine Sun, an attorney with the ACLU immigrants’ rights project. “We anticipate we will be filing notices of claim and lawsuits on behalf of folks who have been adversely affected by SB 1070,” Sun said.

A recent Arizona Daily Star analysis of thousands of records from 13 Southern Arizona law-enforcement agencies revealed a patchwork of immigration-enforcement policies and data so incomplete there’s no way to determine how police are implementing the law or whether they are committing the systemic civil-rights violations opponents feared when the law was passed.

In the case that prompted Friday’s claim, Agustín Reyes and Arturo Robles were stopped on Oct. 8 by a TPD officer near Southside Presbyterian Church, 317 W. 23rd St. The church has a long history of immigration-related activism.

Officer Fabian Valdéz noticed that the light on the license plate of the 1999 Ford van Reyes was driving was out near East 22nd Street and Herbert Avenue, a police report said.

He waited for “several minutes” after the van turned into a private parking lot and was about to resume patrolling when Reyes turned out in front of him.

Valdéz followed the van to West 22nd Street and South 10th Avenue, pulling it over near the church.

Reyes and Robles were unable to provide identification and refused to answer the officer’s further questions, so Valdez contacted the Border Patrol to investigate their immigration status.

It is a misdemeanor under Arizona law to drive without a valid license. Police Chief Roberto Villaseñor has interpreted SB 1070 to require an immigration status check during all arrests, even when a driver is cited and released.

“This is racial profiling, pure and simple,” Sun said in a news release. “Mr. Reyes and Mr. Robles were detained for no other reason than for the police to look into their immigration status and call the Border Patrol. This type of harassment should not be tolerated.”

Villaseñor said in October that Reyes was not targeted based on his race, as the officer who cited him was also Hispanic.

Sun disputes that logic. “I don’t see anything about the officer’s ethnicity that would somehow exempt him from a claim that he racially profiled Arturo and Agustín,” she said.

Border Patrol agents took both men into custody and released them on bond days later. Reyes received a citation for a broken license-plate light and for driving without a license, which he paid in full, the release said. Robles was not charged with any violation.

“At that moment I felt so helpless,” Reyes said in the release. “It’s so difficult to be in that situation, because you assume the police are there to protect people. When this happened, I thought they would call the Border Patrol, and that’s exactly what they did.”

The city has 60 days to respond to the claim.