Paris Hilton's latest run-in with the law began when a motorcycle officer got a whiff of suspicious smoke emanating from a Cadillac on the Las Vegas Strip.
Suspecting the odour was marijuana, the officer stopped the car at 11.22pm on Friday and during a check, police say a bag of cocaine fell out of the 29-year-old socialite's purse.
The officer "followed the vapour trail and the odour of marijuana to the Escalade," said Sgt John Sheahan.
It is the second time this year that Hilton has been arrested on drug possession allegations, although authorities in South Africa dropped marijuana charges earlier this summer.
In 2007, Hilton pleaded no contest to alcohol-related reckless driving and was sentenced to 45 days in jail.
This time, the hotel heiress was with her boyfriend, Las Vegas nightclub mogul Cy Waits, who manages a club inside the Wynn Las Vegas and was driving the black SUV.
As other police arrived and a crowd gathered on the busy neon-lit Strip, Hilton asked to go into the Wynn resort for privacy, Sheahan said.
"Miss Hilton pulled out a tube of lip balm," Sheahan said. "At the same time... a bundle of cocaine in a plastic bag came out of her purse" in plain view of police in the room.
Police Officer Marcus Martin characterised the cocaine as a "small amount," a package of the size usually associated with personal use. Police would not specify the weight of the cocaine or whether marijuana was confiscated.
Hilton was arrested on suspicion of felony cocaine possession. If convicted of the low-grade felony, she would get probation, but any violation of that probation would be punishable by up to four years in Nevada state prison.
Waits (34) was arrested on suspicion of misdemeanor, driving under the influence of alcohol or drugs. Misdemeanors are punishable by up to a year in county jail. Police said he owned the 2009 Cadillac.
Hilton and Waits were booked into the Clark County jail, where Sheahan said Hilton was kept handcuffed on a booking room bench, fingerprinted, photographed and released without bail at about 2.45am on Saturday.
Waits' lawyer, Richard Schonfeld, said Waits posted $2,000 (e1,570) bail on Saturday and was released that afternoon.
Sheahan said Hilton was not taken to a cell and received no special treatment on a busy Friday night and Saturday morning at the jail. He said release without bail was common in such cases.
Hilton's attorney, David Chesnoff, said he was still gathering facts about the arrest.
"This matter will be dealt with in the courts not in the media, and I encourage people not to rush to judgment until all of the facts have been dealt with in a court of law," he said.