PARIS—Police were conducting a frantic manhunt Monday for a gunman suspected in two shootings and a brief hostage-taking that stunned Paris and which authorities were treating as possibly terrorist-related.

One man, a photographer's assistant, was in critical condition after being shot in the chest at the offices of a national newspaper, police said.

Authorities seemed to be at a loss as to the gunman's identity or motive, and appealed for witnesses to come forward to help track him down. The 65-year-old man who had been briefly taken hostage told police that his assailant claimed to have a hand grenade and said he had just gotten out of jail.

"Our main lead is that there is a single perpetrator," prosecutor François Molins told a news conference. He said his staff was investigating any possible motivation, including terrorism.

Prosecutors showed photos taken from surveillance video showing a man with a pump-action gun, and said he was suspected in another gun-related incident last week at an all-news television network as well.