[Updated at 2:05 p.m.] "This was not part of a terrorist attack," said Deputy Commissioner of Police Paul Browne. "He wanted to find a place in the tunnel where no one would find a body."

Subway workers saw the man walking on the tracks between Brooklyn's Borough Hall station and the Bowling Green station in Manhattan at 5:30 a.m. ET, Cavitolo said. They called police when they realized he was not an employee.

The man was carrying flares, water bottles, and a one-quart canister that contained sodium cyanide, he said.

Investigators believe he planned to drink the cyanide to kill himself, he said.

Browne said subway passengers were never in danger.

"Sodium cyanide does not have explosive capability, but is extremely toxic, especially when mixed with water," he said.

The man had no contact with passengers, he added.

[Posted at 12;15 p.m.] New York Police arrested a college student who they say carried cyanide and flares into a Manhattan subway Friday morning.

The 20-year-old man, who was wearing a fake transit system vest, told police he planned to kill himself, New York Police Detective Joseph Cavitolo said.

- CNN's Susan Candiotti, Hussein Saddique and Chris Kokenes contributed to this report.