Sandra Bullock Stalker in Standoff with Cops at Home in L.A.

1:10 PM PT -- Law enforcement sources tell us ... Corbett has died during the standoff. We're told it appears to be a self-inflicted gunshot wound.

Looks like Sandra Bullock had good reason to fear her stalker being released from a mental health facility last year, because he's now causing trouble once again ... TMZ has learned.

Law enforcement sources tell us ... Joshua Corbett is currently in a standoff with cops at his L.A. residence. We're told officers went to his place Wednesday morning to arrest him for violating his probation, but instead of surrendering peacefully ... he threatened harm on them and barricaded himself in his home.

As you may recall ... Corbett was given 5 years probation for stalking Bullock and breaking into her house in 2014 and ordered to get mental health treatment.

Our sources say S.W.A.T. has been called to the scene and the streets around it have been blocked off while they try to resolve the standoff with Corbett.

We broke the story ... Bullock was alarmed when her stalker was back on the streets last June after being released from a mental health facility much sooner than she expected. She got a restraining order requiring him to stay 200 yards clear of her until June 2022.

It's unclear what probation terms Corbett violated to trigger the arrest warrant Wednesday.