WASHINGTON — A new report by an immigration watchdog finds that the United States’ largest federal law enforcement agency rarely punishes its agents for their mistreatment of immigrants and American citizens.

The report by the American Immigration Council found that 97 percent of abuse complaints lodged against Border Patrol agents and Customs and Border Protection officers resulted in no disciplinary action once an investigation had been completed. Those included a complaint from a pregnant woman in El Paso, Texas, that she had miscarried after a Border Patrol agent kicked her in the stomach, and several complaints from women that they had been forced to bare their breasts while in custody.

The survey also found that many complaints against U.S. border agents take years to resolve. The council reviewed 809 complaints filed in the three years from January 2009 to January 2012. But of those, only 485 had been investigated and resolved. The remainder are still under investigation, including a nearly 5-year-old allegation of forced sexual intercourse lodged on July 30, 2009.

Among the cases that were still “pending investigation, the average number of days between the date the complaint was filed and the last record date provided in the data set was 389 days,” the report said.

Spokesmen for Customs and Border Protection, which includes both Border Patrol agents and customs officers at points of entry, did not respond to requests for comment.