A woman who claims to have purchased the winning ticket in a $26m (£18.5m) California Lottery draw says she left it in a trouser pocket and destroyed it in the wash, US media report.
The winning SuperLotto Plus ticket was sold at a convenience store in the Los Angeles suburb of Norwalk in November.
The woman, who has not been named, was reportedly captured by CCTV purchasing a ticket at the store at the time.
But a deadline set for claiming the substantial prize passed on Thursday.
Lottery organisers say the win must be redeemed within 180 days of the draw, which took place on 14 November.
A member of staff at the store in Norwalk, Esperanza Hernandez, told California's Whittier Daily News that a woman had visited on Wednesday claiming that she was the prize-winning ticket-holder, but that she had lost the ticket and feared she had accidentally added it to a clothes wash.
A spokeswoman for the California Lottery, Cathy Johnston, told the newspaper that the store footage would not be enough to substantiate the woman's claim, and that "compelling substantial proof you were in possession of the ticket" was required.
However, a copy of the video has been shared with the lottery organisers and the claim is being investigated further, Ms Johnston confirmed.
If there is no recognised winner of the jackpot, cash would be distributed among California's public schools, the organisers said in a statement. The retailer that sold the winning ticket has received $130,000 as part of the lottery's standard procedure, the statement added.
The winning numbers for November's unclaimed draw were 23, 36, 12, 31, 13, with a so-called mega number of 10.
It is extremely rare for winning ticket-holders in the California Lottery not to come forward, the Los Angeles Times reported, citing the competition's organisers.
It is hardly the first time people have claimed to have bought winning tickets that they were later unable to produce.
Back in 2016 BBC Newsbeat tested a ticket in the washing machine after a UK woman said she had left her winning numbers in a pair of dirty jeans.
The ticket disintegrated.