Lawson said initial testing showed a possible match between Thomas’s DNA and evidence in a 1999 rape in the city.
Greenbelt is located in Prince George’s County, where several other attacks linked to Thomas have occurred.
Thomas is charged with three rapes in Virginia and has pleaded not guilty to another in Connecticut.
A prosecutor has said Thomas probably will face trial in New Haven on a first-degree sexual assault charge before he is tried in Virginia.
Lawson said police in Greenbelt would not file new charges against Thomas unless the lab tests proved a definitive DNA match directly connecting him to the 1999 case.
Detectives submitted all their evidence to the lab after getting a hit in a DNA database.
Thomas, an unemployed truck driver, was arrested in March in his hometown of New Haven after authorities said DNA confirmed he was the so-called East Coast Rapist.
Authorities had mounted a large-scale public outreach campaign, putting up electronic billboards in the states where the attacks occurred and in neighboring states. DNA from a cigarette that police saw Thomas discard after leaving a New Haven court was used to confirm that Thomas was the man wanted in the attacks, a prosecutor has said.
Thomas has been suspected of rapes dating to 1997, but he has told investigators that he began committing sexual offenses six years before that, according to law enforcement officials familiar with the case.
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