(CNN) -- Detroit police are investigating the death of two women whose bodies, burned beyond recognition, were found in the trunk of a car on fire early Christmas Day.
Less than a week earlier, the bodies of another two women were found in the trunk of a car parked at a vacant dwelling, Detroit police said.
Three of the four victims were linked to online ads that "specifically dealt with prearranged adult dating services," Police Chief Ralph L. Godbee said at a Monday afternoon news conference.
"We felt it is imperative to alert the public that deciding to meet unknown persons via the Internet can be extremely dangerous," Godbee added.
In the Christmas morning incident, "the victims have been identified tentatively and are black females, ages 28 and 29," Godbee said.
The bodies found on December 19 were those of two African-American women aged 23 and 24, and neither body showed outer signs of trauma, according to Godbee.
"We are awaiting the medical examiner's final report for the determination of the cause of death for each of the females." Godbee added.
Godbee identified the website carrying ads that three of the victims were linked to as backpage.com.
The website did not immediately respond to an e-mail from CNN Monday afternoon seeking comment after Godbee's news conference.
Backpage.com told CNN in 2010 that it promptly responds to law enforcement inquiries, and said the site includes links to help users notify the National Center for Missing and Exploited Children if they identify potential abuses.
In addition, the website includes links for the FBI, Federal Trade Commission, and the Polaris Project, a program set up to help victims of human trafficking.