Source: Philip Seymour Hoffman's death being treated as an accident

NEW YORK – Philip Seymour Hoffman's sudden death this weekend is being treated as an accidental overdose, a source close to the investigation tells FOX411.

The source said Hoffman injected himself, but it is not believed he committed suicide.

A rep for the medical examiner would not confirm the ruling, but said the 46-year-year-old actor's autopsy is being performed today.

Police sources also tell Fox News they found considerable evidence of drug use in Hoffman's apartment, including 50 glassines -- small plastic bags -- some of which were empty, and many of which still contained drugs, used and unused hypodermic needles, and multiple prescription drugs.

Hoffman's family has hired security around the apartment building he was living in to make sure no one can get inside, a source said.

Hoffman was living in a rental apartment a few blocks from the building where his partner Mimi O'Donnell and three children lived. He had reportedly separated from O'Donnell some months ago, and was supposed to be picking up his children from her when a friend found him unreponsive in his apartment with a needle stuck in his arm on Sunday morning.

Police investigating his death questioned the manager of a neighborhood grocery Monday, asking if the store had any surveillance video of the actor.

FOX411 first reported that Hoffman had been spotted at the grocery around 10:30 Saturday morning looking "gray" and purchasing tissue and soda.

Investigators are also trying to track down surveillance video at a nearby bank where a man told cops he witnessed what he believed to be Hoffman buying drugs from two men several hours later on Saturday night.

The man told investigators Hoffman looked "very sweaty" and "like s**t."

Hoffman's trip to the ATM at about 8pm is believed to be around the last time he was seen alive, sources said.

Sources said police are looking into whether Hoffman had injected a lethal batch of heroin behind hundreds of recent deaths across the country.

E-mail tips to Jana.Winter@foxnews.com.