HOUSTON (AP) — Police say four suspects have been arrested in the investigation of at least 30 armed robberies and two sexual assaults in the Houston area since June.

Houston police on Wednesday announced the men — ages 17, 18 and two suspects who are 21 — are charged with aggravated robbery with a deadly weapon. One suspect has also been charged with sexual assault.

Police say in most of the cases the robbers carried handguns or semi-automatic rifles, demanded property from victims and threatened to harm them.

Investigators say the string of robberies happened in Houston, Missouri City, Pasadena, Sugar Land and Montgomery County since June 21.