PORTLAND — The Portland Police Department is calling on the public to help it solve a missing person case.

Police say that Nathan G. Bihlmaier, 31, of Cambridge, Mass., has not been seen since he left an Old Port bar about 12:30 this morning.

Bihlmaier and two male friends came to Portland this weekend to celebrate their upcoming graduation from Harvard Business School.

He was last seen leaving the Ri Ra Irish Pub on Commercial Street by his companions. Portland police were able to track his movements for about 40 minutes after he left Ri Ra before his cellphone went dead.

“It’s an unusual one,” said Lt. Gary Hutcheson. “He is a well established individual from a strong family. He has a wife, who is expecting their first child. There is no reason he would (purposely) go missing,” Hutcheson said.

The Portland police dive team searched the water near the Casco Bay Lines ferry terminal Sunday afternoon, but could not find any evidence to suggest Bihlmaier might have fallen into the bay.

“We are kind of at a loss,” Hutcheson said Sunday night.

Anyone with information concerning Bihlmaier’s whereabouts should contact Portland police at 874-8479.