(CNN) -- Police pressed their case Friday against a Missouri man who, they say, tested positive for HIV in 2003, then had more than 300 sexual partners -- saying later he didn't tell them about his condition because of his "fear of rejection."

Dexter, Mo., police Det. Cory Mills said he filed a probable cause statement Friday for two more alleged victims who wish to seek charges against David Lee Mangum. This could lead to more counts against the 36-year-old Mangum, who is already facing a felony charge accusing him of exposing one of his former live-in partners to HIV.

The scope of what happened and what's to come remains to be seen -- both in terms of the breadth of legal trouble Mangum will face and the hundreds of lives he might have forever altered, directly and indirectly.

To the latter point, authorities in the rural southeastern Missouri county where Mangum lived are urging anyone with reason to suspect they could have been infected to stop their sexual activity and get tested.

"Due to the initial exposure containing 300 or more individuals over an extended period of time, each of which could themselves have multiple sexual partners, this situation should be a serious concern," Stoddard County Prosecuting Attorney Russell Oliver said in a statement.

Oliver added it was especially important action be taken by anyone in the area who had met for a sex act through the Craigslist website's "men seeking men" section.

That's how the victim said he first connected with Mangum in October 2012, according to a criminal complaint obtained Friday by CNN. The pair then had unprotected sex -- though only after the victim told police he "specifically asked if (Mangum) had any disease and Mangum replied no," the document says.

The next month, Mangum and the younger man moved in with each other. Except for a 16-day stretch in December, they remained live-in partners in Dexter, a city of about 8,000 people some 160 miles south of St. Louis, until June 2013.

It was then the younger man, now 29, told police he ended the relationship "because he discovered Mangum was cheating on him."

Sometime later, according to the complaint, that man got a call from a woman who had lived with Mangum in 2011. She told him that Mangum revealed to her he'd been HIV-positive since 2003. After that, the victim took a test at the Stoddard County Health Department that showed he also had the virus.

The suspect confirmed he had tested positive in Texas to the victim and later to police, officials said. That diagnosis didn't stop Mangum from having hundreds of sexual partners -- including 15 to 20 instances of unprotected intercourse, sometimes involving a third man, with the victim tied to the charge issued this week -- according to the criminal complaint.

The same day he talked with the victim, Dexter police detective Mills found Mangum, who agreed to be interviewed and waived his Miranda rights. The suspect confirmed what his former partner had said, saying he didn't tell those he had sex with about his HIV diagnosis because of "fear of rejection," the complaint says.

While authorities have tracked down some of Mangum's sexual partners -- 50 to 60 of whom lived in Stoddard County -- "countless others remain unknown," Oliver said.

Mills said the suspect may not be of much help, because Mangum "usually only knew his partners' first names." Many of them were in the Dallas area, where Mangum spent some time, according to the Dexter police detective.

"Mangum indicated all his sexual partners were white males," Mills noted.

Knowingly exposing someone to HIV without their consent is a felony under Missouri law that can bring a prison term up to 15 years. Infecting someone can bring a life term.

Mangum's bail was set at $250,000 during his arraignment Thursday in Stoddard County.

CNN's AnneClaire Stapleton contributed to this report.

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