Search-and-rescue crews were seen pulling a body from the Charles River between Boston and Cambridge Wednesday.
Authorities from both cities had been searching the river for at least two hours near the Boston University Bridge.
Authorities haven't released any information about what they believe happened to the man.
Witnesses said they saw a shirtless man struggling in the river Wednesday morning. They tried to get to the middle of the river to rescue the man but lost sight of him in the water.
"I was just on a run and noticed there was somebody in the water when I got to the part of the Esplanade that goes around the B.U. boathouse," Hannah Cashen said.
Rob Carey was biking to work when he "saw someone thrashing" in the water, he said.
Both knew something wasn't right.
"And we started to try and call him, 'Hey, are you good? Are you good?'" Cashen recalled.
They knew they needed to act fast.
"I knew I wasn't a good enough swimmer to help," said Carey. "But there were these boats scattered all over, so I said, 'Let's go get one of those boats'."
They untied a boat at the B.U. Sailing Pavilion and paddled toward the man in the river.
"We wanted to throw the rope in and find him but we didn't," said Carey. "It was just black water."
They lost sight of him as the man went under.
"We didn't realize it was the last time we saw him," said Cashen. "We couldn't see his head anymore."
By then, divers and a tech rescue team were arriving, and the two Good Samaritans were able to point them in the right direction.
"That's a good assistance for us," said Deputy Boston fire Chief Michael Doherty. "They were able to give us a location as far as where we could start our search.":
After over two hours, crews finally located the man in about 10 to 12 feet of water.
"I thought we were going to be able to get there," said Carey. "I really did, and to get there and see nothing was just incredibly disheartening."
It's unknown why the man was in the water in the first place. He hasn't been identified.