From cis.org:

Rep. Duncan Hunter (R-Calif.) made news recently with his claim that ISIS militants have been caught crossing the southern border into the United States. It was a startling assertion, but not one to be lightly dismissed given the recent evidence in the Rio Grande Valley of Texas, and elsewhere, that the border is not nearly as secure as administration officials would like us to believe.

Southern Border
At least one Texas lawman gives credence to Rep. Hunter’s statement. Midland County, Texas, Sheriff Gary Painter, pointing to a porous, “open” border says physical evidence at known crossing areas suggests that Muslims have been crossing illegally and that “there was some activity for the sheriffs along the border to be on alert.”

http://cis.org/cadman/have-terrorists-crossed-our-southern-border