Authorities were investigating the source of a mysterious gas-like odor Monday that stretched across a large part of Manhattan and prompted officials to suspend some commuter rail service to New York. The Fire Department began getting calls about the odor around 9 a.m., said spokesman Tim Hinchey. The cause was not immediately identified. The Port Authority of New York and New Jersey suspended PATH train service between New Jersey and the Madison Square Garden area in Manhattan. Across the Hudson River in Jersey City, Maria Pignataro, the spokeswoman for Mayor Jerramiah T. Healy, said officials were told the odor in their city was due to a gas leak in the Chelsea section of Manhattan. No casualties were reported. Mayor Michael Bloomberg said there was no indication that the air was unsafe to breathe, and he said sensors did not show an unusually high concentration of natural gas.