The surveillance video shows William McQuain, dressed in shorts on an October Saturday, rolling around the parking lot of a Germantown self-storage warehouse on the wheels in his sneakers.
"He was behaving like a typical 11-year-old," said Montgomery County Police Chief J. Thomas Manger. "Gliding up and down the pavement."
By the end of the day, Oct. 1, police said, Lopez would also kill William and toss his body in the woods.
For days, police and volunteers conducted a massive search in hopes the boy would be found alive. Friends gathered for a vigil, and his football buddies put stickers on their helmets: "William McQuain. We ♥ U."
But Tuesday morning, hope turned to mourning when a police officer found the boy's body in a wooded area just off a Clarksburg road. Detectives had focused on that area after tracking Lopez's cellphone use.
"This confirmed our worst fears," said Jeff McDermott, who coached William on a Little League All-Star team. "He was a great kid, a great player. He did everything we asked him to do."
Lopez and Jane McQuain had a long and troubled relationship, but only days earlier the two had taken William on a vacation to Ocean City. A friend said Jane McQuain called from the beach, saying that she and Lopez had argued and that she was scared.
Exactly what happened that weekend might never be known. Lopez, who was arrested Thursday in North Carolina, has refused to talk to investigators, police said. The police account of what happened to William and his mother is based on the evidence that had been gathered. Lopez has been charged with murder in the slaying of McQuain, 51.
Montgomery police were first called to McQuain's Germantown apartment last Wednesday by a friend who was worried after she had been out of touch. Officers who climbed a ladder and crawled through a window found McQuain dead in her bedroom. There was no sign of William, nor any sign that he had been hurt in the apartment.
But there were plenty of signs of him and accomplishments: Photos and certificates hung throughout, an indication of a single mother whose life revolved around a quiet, sharp kid with a sweet disposition.
"The apartment was like a shrine to him," a law enforcement source said.
An acquaintance of McQuain's told police about a man seen going in and out of her apartment, according to court papers posted online by WUSA-TV (Channel 9). A neighbor saw the man load a TV box into McQuain's sport-utility vehicle. That man was Lopez, court papers said.
Police found Lopez last week after McQuain's 2011 Honda CRV had been in an accident in Charlotte, according to the court papers.
The woman driving the Honda said she was Lopez's girlfriend and told police that she was driving her boyfriend's mother's car and that the mother's name was Jane.
The next day, Oct. 13, authorities found Lopez and the woman at a hotel in Charlotte. Lopez's girlfriend told police that he had taken an Amtrak train to Philadelphia in September to visit family, according to the warrant. He returned driving the Honda and brought back a large television in a box and an Xbox video game system, the warrant says.
Officers found one of McQuain's credit cards and a box cutter in the hotel room, the warrant says.
Montgomery detectives went to Charlotte to interview Lopez, but he refused to cooperate, said Capt. Paul Starks, a spokesman.
So officers tracked Lopez's movements in Montgomery using cellphone records, according to sources and court records. Dozens of officers and volunteers began searching woods in the Clarksburg and Damascus areas. On Tuesday, a search dog led an officer to the body of a boy.