At first, police thought she might have been the best little drug runner in town.

The 4-year-old girl walked into school Tuesday morning, her blue book bag clasped in her hand. In the hallway, she accidentally dropped the bag and out fell 13 wax paper envelopes containing a "suspicious white powdery substance," as police like to say.

An alert parent volunteer spotted the envelopes, and school officials notified police. Their suspicions were confirmed. The substance was heroin -- 1 to 2 percent pure "standard street strength" dope, police said, worth $35 to $55 an envelope.

Was the girl, whom the police refused to identify, either a witting or an unwitting courier of illicit narcotics?

"No on both counts," said Baltimore Police Sgt. Mike Bass today.

Officers interviewed the girl, a prekindergartner at the Dallas F. Nicholas Elementary School just north of downtown Baltimore. They came up with little information at first, Bass said, except it became clear that she "did not know how [the heroin] got in her book bag."

"We were working under probabilities," Bass said. "It was not probable that a stranger put it in her bag. So that meant there was a greater probability it was a member of her family or a friend of the family."

Police interviewed the girl's mother and father. "They had absolutely nothing to do with it," Bass said. But the mother mentioned that her 18-year-old brother, just paroled from prison on a drug conviction, had come to her home Monday, asking to stay for the night, Bass said. She reluctantly agreed, and the next morning after the child had gone to school, the brother asked the mother if she had seen a "blue bag" in the house, according to Bass.

When she explained that the bag was her daughter's school book bag, the brother became "extremely upset" and left the house, Bass said.

Theorizing that the brother had dropped the heroin in the book bag as "temporary storage," police assigned plainclothes officers to monitor some of the popular hangouts in the neighborhood, hoping to find him, Bass said.

"Sure enough," he said, the brother "showed up . . . and we arrested him today without incident." He said officers seized an additional five envelopes of suspected heroin when searching the suspect. Charged with possession of heroin with intent to distribute was William Mayburn Butler, of no fixed address.