Kansas City, Mo. ? A federal court judge Tuesday denied the release of three Chinese citizens accused of bringing women from their home country to the Kansas City area for prostitution.

U.S. Magistrate Judge Robert Larsen declined to set bond, saying he has concerns about releasing them because they are not U.S. citizens.

All three – Ling Xu, 45; Xu’s boyfriend, Zhong Yan Liu, 35; and Xu’s son, Cheng Tang, 21 – have been living in Overland Park, Kan. They were arrested last week on charges of transporting people across state and national borders for the purpose of prostitution.

Their attorneys declined comment Tuesday.

A fourth defendant, Hongmei Madole, 31, was arrested last week on the same charges and remains in custody, though Larsen set a $50,000 cash bond for her. Court records show she has been living in Olathe, Kan., and is married to a U.S. citizen.

Federal prosecutors accuse the four of operating massage parlors that were fronts for prostitution. The businesses operated in Overland Park, Olathe and other communities in Johnson County, Kan.

Larsen on Tuesday sent the case to a federal grand jury, which will decide whether to indict Xu, Liu, Tang and Madole.

The case resulted from an 18-month investigation that remains in progress.