Fears are growing for a grey whale entangled in a fishing net off the coast near San Francisco.
The nine-metre (30ft) long creature was captured on video swimming with its tail trapped in a massive gill net.
It was first spotted off Laguna Beach in southern California in March, when rescuers attached two red buoys to the net to make it easier to see.
However, marine rescuers, who have been working this week to try and cut the whale free, told local media that a tracker attached to the animal had broken off and it was now missing.
They have also had to suspend their search due to an incoming storm.
Earlier this week, rescue crews attempted to cut the net from the whale's tail. But, whenever they got close, the whale became aggressive and they were unable to continue.
Justin Viezbicke, from the National Oceanic and Atmospheric Administration (NOAA), said the animal "became very reactive".
On the loss of the tracker, he told The San Francisco Standard: "We still haven't figured out why, but something happened out there where the satellite tag buoy broke off."
The team were reportedly able to source a new tracker by Wednesday, but by that time the whale had disappeared.
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Officials from the NOAA and Marine Mammal Center in Sausalito, California, are now searching for the whale north of San Francisco, as it is thought to be continuing its migration.
Every spring, grey whales migrate 5,000 miles from birthing waters off California and Mexico to feeding grounds in the Arctic.
Kathi George, from the Marine Mammal Center, said the rescue team will try again to cut the net, or at least attempt to attach another satellite tag, if the creature is spotted again soon.
She said: "Our goal is to retrieve the gear that's on the whale, so we can learn more about the entanglement and how it happened, so we could use that to inform risk reduction efforts."
However, officials said time was running out to save the whale.
Ms George, speaking to Fox KTVU, said: "It is not able to forage... so I'm pretty confident that it is not eating."
In a post on X, the Marine Mammal Centre said on Thursday: "Due to worsening weather conditions, there will be no response tomorrow [Friday], but we continue to work with partners... to strategise, plan and continue shoreside monitoring efforts ahead of the storm system due this weekend."