Britney Spears DUI, Cops Suspect Drugs and Alcohol

3:10 PM PT -- Law enforcement sources are now clarifying ... cops suspect Britney was under the influence of substances, in addition to alcohol, but say there were no drugs found in her car.

The CHP press release says "chemical test results are pending" ... were told that only means they're testing her blood for drugs and alcohol.

Britney Spears may be in trouble for more than just allegations of drinking and driving ... TMZ has learned.

Sources familiar with the investigation tell TMZ ... in addition to suspicion of DUI, there was also an unknown substance found in her car, which will be part of the arrest.

We're told the substance is now being tested.

As we reported, Spears was arrested for DUI last night in California, and was released from jail around 6 AM today, according to Ventura County inmate records.

The 44-year-old Grammy winner has had a drug-addled past. In a 2007 custody battle with ex Kevin Federline, a judge found "there is a habitual, frequent, and continuous use of controlled substances and alcohol by" Spears.

Since then, the "Toxic" singer has sought mental health treatment through multiple rehab stints.