EXCLUSIVE

Lindsay Lohan is now formally a suspect in the theft of $100,000 worth of watches and sunglasses ... law enforcement sources tell TMZ.

As TMZ first reported, Lindsay was at the Hollywood Hills home of Sam Magid a week ago when Sam called the LAPD and reported a jewelry theft. We're told Lindsay has been at the house frequently, including at an all-night house party a week ago Sunday.

Law enforcement sources tell us ... Lindsay and her assistant Gavin Doyle are both suspects in the theft.

Now here's the twist ... We're told Sam has recanted his story and now says nothing was stolen. But our sources say cops are not closing out the investigation because they believe they have independent witnesses who can finger Lindsay and Gavin as the thieves.

Sources tell us ... LAPD detectives contacted the lawyer for Lindsay and Gavin today, asking for an interview with the two suspects but the lawyer rejected the request.

It's unclear why Sam -- who is insanely wealthy -- suddenly changed his tune.