The 27-year-old man Prince George's police say was an elusive serial killer was busted by federal authorities a year ago for selling firearms from the parking lot of his UPS job, according to sources and court records.

Special agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives began investigating the man last year after they received a tip that he was showing off assault-style weapons from the trunk of his blue Toyota Camry at the United Parcel Service customer service facility in Landover, according to court documents filed in federal court in Greenbelt.

Police have portrayed the man, whom The Washington Examiner has chosen not to identify because he has not been formally charged with the murders, as a "brilliant" adversary.

But his history is that of a mundane career criminal with gun, burglary and child porn charges on his record.

Prince George's County police said they have connected him to the two mother-daughter slayings in Largo last year. Police Chief Roberto Hylton is predicting that the Upper Marlboro man will go down as one of the "most notorious criminals in America."

Police said the man killed Delores Dewitt, 42, and her 20-year-old daughter, Ebony. Their charred bodies were found March 16, 2009, in a burning car in Largo, just two months after another mother and daughter, 45-year-old Karen Lofton and 16-year-old Karissa, were found shot to death in their Largo home.

Police said the man fooled an FBI profiler who ruled out a connection in the two crimes. The Washington Post quoted police sources as saying the man was a "brilliant" criminal, and holder of two master's degrees.

But his father denied that to a reporter for WUSA 9, saying he only had one B.S. degree, from the University of Maryland University College. A school spokesman told The Examiner that the suspect did not have a master's degree, but graduated in 2008 with a bachelors degree in information systems management.

After his arrest for the firearms charges, authorities searched his home

where they found photos taken during a home invasion, according to law

enforcement sources.

Detectives also found child pornography on a computer or cell phone.

According to charging documents, the man used a gun to force a minor girl

into having sex and took her photos.

Police do not believe the killer knew his victims. They are investigating whether he used his position at UPS to access information and find his victims.

The suspect remains behind bars on federal charges, including carjacking and child pornography, which have since been tacked on to the weapons charges.

Neighbors in the tree-lined middle class community where the suspected killer grew up were shocked to learn the news Wednesday. The mother-daughter murders happened nearby.

"It's freaky to think that the suspect lives right here across the street," said neighbor Diedre Moore.

Joe Flechas contributed to this report.