According to Round Lake Park Chief George Filenko, Best's body was discovered Monday evening inside of her green Pontiac minivan, which was parked in the lot of an apartment complex. Family members were notified of the discovery shortly thereafter, he said.
"We were hoping for the best. Unfortunately, this concluded not quite the way we wanted," Filenko told reporters during Tuesday's news conference.
Police say drugs and alcohol were involved in the woman's death, and they are investigating the possibility of an overdose. They say Best had a history with alcohol, but it was not clear if she had a history with drugs. Authorities are not ruling out the possibility of foul play in the woman's death.
"At this point, there's nothing conclusive until we get the autopsy results back, and then we can direct our attention towards something more precise," Filenko said.
Filenko said police are talking to two persons of interest. He also said authorities are working the state's attorney's office and that there is a chance charges could be filed under an Illinois statute against concealing a death.
Best, 34, had not been seen in more than a week. She went missing April 15 after she dropped off her 15-year-old daughter at a friend's home at the Rosewood Apartments in Round Lake Park. Surveillance video showed her minivan coming and going.
Police say evidence indicates Best died in Waukegan the same day she went missing and was driven later to the parking lot of the Gray's Point Studios near Route 120 in Grayslake, where her body was found.
Investigators said they had not had many leads in the case because Best did not have a cell phone or credit cards and was, therefore, difficult to track. Filenko called the case 'baffling' from the beginning. The FBI had joined in the local police investigation.
Filenko also said Tuesday that Facebook place a significant role in locating Best. He said police received tips and leads from Best's friends that detectives were able to investigate.
"The family put a substantial amount on information of Facebook, and in turn we were contacted by friends, high school acquaintances that were giving us all sorts of information we were following up on," said Filenko.
"She would always call me and tell me where she was," husband Clinton Best said during an interview Monday. "She was, you know, pretty responsible about thinks like that. It's completely unlike her to just disappear this."
Filenko said Clinton Best was devastated at learning the news of his wife's death. Clinton Best posted a message on Facebook Monday night thanking everyone for their prayers and help in finding her.
Friend Christine Johnson tearfully paid tribute to Melissa Best
"[I thought] that she was taking a break from her family and that the minute all of this came out this way, she was going to call and tell them she was safe," said friend Johnson. "She was a great mom. She was a great wife. She was a great person. The nicest person in the world."
According to authorities, the investigation into Best's death continues. Best's minivan is part of the death investigation and has been processed for evidence, police said.
An autopsy has been scheduled for Wednesday. Filenko said the results of toxicology tests would be available seven to 10 days later. Those results could help determine whether or not the case becomes a criminal investigation.