Investigators are trying to determine what a man was doing on the tracks just before he was struck and killed by a Long Island Rail Road train early Thursday in Islip.

The incident, which occurred at about 2 a.m. west of the Islip station, disrupted service on the Montauk branch, forcing the cancellation of at least one train and delays to others, the railroad said.

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Officials said Metropolitan Transportation Authority police are investigating the fatal strike on the tracks between Grant Avenue and Saxon Avenue in Islip. The man, whose identity has not been released, was struck by an eastbound train. The site was not near an authorized crossing.

In an advisory to Montauk branch customers, the railroad said that the 5:39 a.m. train from Montauk to Long Island City was canceled due to the late arrival of equipment in Montauk as a result of the fatal incident. Customers were to be accommodated by a bus from Montauk to Speonk, where officials said they could an connect to a westbound train.

Officials also said the 7:12 a.m. train from Speonk to Hunterspoint Avenue would make added stops at Hicksville and Mineola, while the 4:37 a.m. train from Speonk to Babylon operated 15 minutes late through Patchogue and the 5:08 a.m. train from Speonk to Penn Station operated 19 minutes late out of Speonk.