Reported five days ago by a fishing crew, the object hasn't been seen again even though the U.S. Coast Guard and National Oceanic and Atmospheric Administration are looking hard.

"The dock hasn't been located again since the initial report," NOAA spokeswoman Keeley Belva told NBC News. "We have been doing trajectory models, but given that the first sighting was several days ago, it becomes increasingly challenging to anticipate where and when the dock may show up."

Crew aboard the "Lady Nancy" sent the U.S. Coast Guard a photograph of the object, saying it was taken about 16 nautical miles northwest of Grays Harbor, Wash.

The Coast Guard said Monday it was broadcasting an alert to mariners but that after five helicopter searches covering 317 square miles it hadn't spotted the flat, dark object.

"There could be many factors in why it hasn't been seen," Belva said when asked if it might have sunk. "The weather has been pretty stormy and the seas have been rough. As you can see from the picture, it is hard to see even in better conditions."

The photo shows an object similar to a Japanese dock that washed ashore in Oregon last June. The 66-foot-long dock is the largest tsunami item to have made it to the West Coast.

Another dock was seen off Oahu, Hawaii, in September but Belva said it wasn't known if that was Friday's sighting.

Japan estimates the 2011 tsunami swept about 5 million tons of debris into the Pacific, and that two-thirds of that sank quickly. Some of the remaining 1.5 million tons are heading for West Coast shores.