The man who killed four people at a board meeting in Philadelphia's Navy Yard appeared to be upset about something going on in the company, police say. Four men were killed and a fifth was critically wounded in a shooting in an office building in Philadelphia's Navy Yard on Monday (February 12) evening, police said. The gunman shot four men, at least one of whom had been bound with rope and tape, killing three of them, according to local media reports. The fourth victim initially survived the assault and was taken to a local hospital. The gunman also shot at police, who returned fire. The shooter then killed himself, police said. Deputy Commissioner Richard Ross said the shooting happened during a board meeting for one of the businesses in the Navy Yard Business Center, a new development on the south side of Philadelphia. "This male who we believe was the shooter was actually inside this meeting. It's really sketchy right now but it appears he was upset about something that was going on in the company. He got into an argument with several people at that board of directors' meeting and subsequent to that he fired several shots at these males in question," said Ross. Police are investigating if the gunman may have been an investor in the company. Philadelphia's homicide rate rose to a nine-year high of 406 in 2006, giving it the highest murder rate per 100,000 people among the 10 largest American cities.