Police in Kansas City, Missouri, have determined that a 20-year-old college student who was found dead in a river in her submerged car in March accidentally died of hypothermia and drowning — but that alcohol and drugs played a role in her death.

Toni Anderson‘s body was found on March 10 after her vehicle was pulled from the Missouri River with her still inside. She had vanished on Jan. 15 after she left work at the Chrome club in Kansas City, just after 4 a.m.

She was on her way to meet up with a friend, but police say she never made it.

“The manner of death is accidental and the contributing factors are ethanol, cocaine and amphetamine intoxication,” Kansas City police said in a statement obtained by PEOPLE.

Authorities said that the investigation is now over.

In the weeks following the accident, officials worked to determine the mysterious circumstances surrounding Anderson’s death. However, a police spokeswoman told PEOPLE in March that no foul play was suspected.

• Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter.

Before she disappeared on Jan. 15, Anderson was pulled over by an officer in North Kansas City, Missouri — a separate jurisdiction from Kansas City — around 4:30 a.m. after an illegal lane change. She was let go with a warning and soon stopped for gas, police said.

In footage of the stop uploaded to YouTube, an officer questions Anderson and tells her to briefly sit in a nearby parking lot to “gather” herself after she gave confusing answers.

At 4:42 a.m. that morning, Anderson texted a friend, “OMG just got pulled over again” — but police said she was likely referring to the stop minutes earlier.

Anderson’s mother, Liz Anderson, declined to comment to PEOPLE on Tuesday after her daughter’s cause of death was released.

But she previously said that police believed Toni’s car may have slid off a slick ramp into the river after her GPS system directed her the wrong way as she was turning around.

“If you look at the boat ramp she drove off of, it looks like a regular road — especially at night, when it’s dark,” Liz said. “The police believe the ramp was icy and that she may have tried to stop, but the car went into the water.”

Toni studied marketing at the University of Missouri-Kansas City and, in her spare time, reviewed music festivals, her family said.

Her mother has described her as “a beautiful, smart, intelligent, hard-working, kind and loving person.”

“I am relieved that it was nothing more than a tragic accident,” Liz said.

“It is horrible,” she said. “But they finally found her, and now we have a bit of closure.”

• With reporting by HARRIET SOKMENSUER

ncG1vNJzZmiolaS9rbGNnKamZ5Ontq6xjq2mp6FdlrulsdGspqdllKe8t7GMnJirZZ2ewLS71KugZqqZq7KzecOemK2gXZi8pK3Ip5xo