The shirt reads "legalize gay" and, in smaller letters, "Repeal Prop 8 Now," referring to the California measure banning same-sex marriage. Earlier this week, the window was smashed.
American Apparel blames someone upset about the company's support of same-sex marriages, News4's Michael Flynn reported.
The vandalism could be related to threatening phone calls to American Apparel stores. As recently as Wednesday, workers at both the Silver Spring and Georgetown locations have received calls threatening them for not taking the T-shirt down.
Montgomery County police are investigating whether the vandalism is a hate crime.
American Apparel will not be intimidated and plans to keep the T-shirts on display, officials said.