When Dana Hamilton was fatally shot in southeast Washington on May 19, the suspected shooter had what would seem an iron-clad alibi.
A tracking bracelet secured to his leg after a previous gun arrest showed him in his apartment, 1.5km away from the crime scene.
But District of Columbia police thought Quincy Green, known as "Q," had tampered with the GPS device.
Instead of cutting off or removing the bracelet, however, which would have alerted authorities, police said Green found another way to dupe the tracker: He took off his leg.
Court documents show that the GPS device was attached to the 44-year-old man's prosthetic limb, which police said he removed and put in a box in the living room of his second-floor apartment before apparently putting on a spare leg and heading outside.
For 72 hours, police said in an arrest affidavit, "the device barely moved," still attached to the leg, even as Green himself moved freely without being noticed by officials tracking him under the District's most restrictive form of pretrial release.
Repeated sightings of Green by officers and witnesses were dismissed, police said, with officials insisting Green was where the GPS said he was.
"I don't understand how someone could put this device on a prosthetic leg," said Sergeant Matthew Mahl, chairman of the Washington police union. "It is frustrating for us as police officers to have one of our defendants released, especially when talking about dangerous crime like guns - and then to know that the accountability for these defendants isn't always up to par."
About 400 defendants awaiting trial in the District are free in the neighbourhood, wearing GPS devices. The trackers are not designed to monitor in real time, but can provide officials with a road map if the defendants abscond while wearing a tracking bracelet. Authorities are alerted should a defendant "disappear" from the grid, cut off or forcibly remove a device, or wander in an area the court put off-limits.
Chris McDowell, director of communications for California-based Sentinel Services, which supplies and fits the bracelets on pretrial detainees in the District, said protocol for Green was "absolutely not" followed. He said the company technician who fitted Green put the device over a sock and apparently didn't realise the leg underneath was artificial.
McDowell said regulations require that the devices be affixed tightly to skin, never over clothing. "We believe it was absolutely human error," he said.
Cliff Keenan, director of the D.C. Pretrial Services Agency, said no similar mistake has occurred in the thousands of GPS bracelets put on defendants. "This is the first instance where something like this has happened, and the results were tragic," Keenan said.
Green's lawyer with the Public Defender Service did not respond to interview requests. Prosecutors declined to comment because Green's cases are pending. Police officials also did not comment.
I don't understand how someone could put this device on a prosthetic leg
Hamilton was shot on a Thursday at about 2.40am local time The arrest affidavit does not disclose a possible motive.
The slain man lived with his mother, 72-year-old Lillie Hamilton, in Oxon Hill, Maryland. He was out of work on disability with a bad heart, his mother said, and on the night before he was killed climbed into a van with friends. He told his mother he was going to hand out religious pamphlets. Hamilton ended up at an apartment complex, where his family had lived in the early 1990s, and where his brother, then 22, had been fatally shot.
Hamilton said that the shooting of her son "was the worst thing that ever happened to me" and that she still doesn't know why it happened. Of the problems over the suspect's GPS system, Hamilton said, "What can I say? That man was supposed to be in his house."
Police said surveillance video shows the man known as "Q," who walks with a limp and wears glasses, with two men near the complex and drinking alcohol on the morning of the shooting. The video shows him holding a gun, firing several times at a man as the man fled, the arrest affidavit says.
On May 25, police said a witness identified Green as the suspected shooter. Police then learned that he had been arrested on a gun charge in April and was confined to his home awaiting trial.
But when detectives asked about Green's locations around the May 19 shooting, police said the pretrial agency insisted that the GPS readings "did not place Mr Green in the area when the homicide occurred".
Police asked for data for Green's locations May 19, May 20 and May 25. "All the coordinates placed the GPS device in the 4200 block of South Capitol Street." Police concluded that "it was obvious that Mr Green and his GPS monitor were not at the same location." Police arrested Green later on May 25 and charged him with second-degree murder on Friday.
They said in court files that they found the GPS on the leg in the box, and two chargers.