A humpback whale that is missing its tail and was spotted in Washington state’s inland waters likely lost its iconic flukes after becoming entangled, possibly in some kind of line or fishing gear, experts say. That loss of the flukes, used for propulsion, would appear to be a death sentence for the creature, last reported as seen in late July.

Jessica Farrer, research director with The Whale Museum on Washington’s San Juan Island, was among those who responded to a sighting of the whale July 23 off a nearby island. The institute, as part of its work, helps respond to reports of stranded or distressed marine mammals and educates boaters on best practices for whale watching. Farrer had previously seen injured humpbacks but not something like this. She described the sight of the whale as heartbreaking.

Experts with whom photos and video of the whale were shared have concluded it likely lost its flukes due to entanglement, she said. But it’s not known exactly what it became entangled in.

There hasn’t been another reported sighting of the whale since that day, she said.