Traffic cones have repeatedly been thrown off the Massachusetts Avenue Bridge onto the Charles River, and authorities are investigating.
It took four hours Friday for a team of engineers to retrieve 131 traffic cones from the thinning ice.
Normally, they inspect bridges underwater in warmer weather. But Friday, they used a lifesaver to hoist them back onto the bridge.
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"It is a little bit of a dicey operation whenever you have ice on a river, it can be dangerous," said Massachusets Department of Transporation Highway Administrator Jonathan Gulliver.
The cones are part of a bicycle safety pilot program launched in November.
Last month, about 50 cone were thrown onto the river.
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MassDOT says it's unclear why someone would do this.
"It's hard to say," he said. "I mean, I don't know what the motivation is of somebody to just do that."
A spokesperson for the state police says there is surveillance video, but they have not been able to identify who is responsible yet.