Riggin, describing information in the charging documents, said Shepley originally told police that he had found Barthlow in the bathtub.
On Monday, Shepley called his mother and told her that he had drowned Barthlow, according to allegations in the charging documents, Riggin said. Shepley's father then called police and Shepley surrendered, she said.
According to the documents, Shepley told police in a taped interview after he surrendered that both men had been drinking vodka and malt liquor before Shepley helped Barthlow take a bath, Riggin said.
The two had discussed having Shepley drown Barthlow, but Barthlow said he didn't want to die that way, said Riggin, quoting from allegations in charging documents. Shepley went downstairs to get a baseball bat, but when he returned to the bathroom, he set the baseball bat down, she said.
After the two talked further, Barthlow's head was slammed against the bathtub's water spigot and he was held under the water, Riggin said the charging documents alleged.
Shepley called his parents, who called police.
A report from the medical examiner's office in Baltimore confirmed the cause of death as drowning and noted Barthlow had a one-inch bruise consistent with a blow to the back of the head, police said.
Police said they had been told that Shepley had been living with Barthlow since July.