The Animal Rescue League of Boston is asking for help finding a white and gray pitbull-mix that was taken for a walk by a potential adopter and never returned to the shelter.

The nonprofit said 3-year-old Nick went missing on Sunday.

“Staff followed all protocols and procedures. ARL has reported the incident to [the Boston Police Department] and is extremely concerned for the safety of this animal,” the rescue league wrote on Facebook Wednesday.

A Boston police spokesperson told Boston.com on Thursday morning that detectives are “actively investigating this incident.”

The shelter is asking anyone who sees the dog, or the man alleged to have taken him, to contact police immediately.