A key witness in the double-murder trial of Aaron Hernandez took the stand and described tattoos he gave the former New England Patriots tight end, ink that prosecutors are seeking to use as evidence of his guilt.

The prosecution hopes that some of the more recent tats by David Nelson, Hernandez’s California-based tattoo artist, can be introduced as evidence that Hernandez killed Daniel de Abreu and Safiro Furtado in a drive-by shooting in July 2012. The former NFL player, who is serving a life sentence without the possibility of parole for the murder of Odin Lloyd in 2013, has pleaded not guilty to the murders.

Hernandez is heavily tattooed, but ink on his arm of a gun barrel loaded with five bullets and a spent shell casing is of interest to prosecutors. Hernandez, prosecutor Patrick Haggan alleged in his opening argument in the Suffolk County, Mass., courthouse, fired five bullets at the victims and got the tattoo eight months after the murders. Nelson also testified that he gave Hernandez a tattoo of a smoking gun muzzle with the words “God forgives” written backward.

“The customer makes all decisions,” Nelson testified as Judge Jeffrey A. Locke, who will determine whether Nelson will testify in front of the jury, was shown a number of photos of Hernandez’s tattoos. His lawyers argued that Nelson’s testimony was inconsistent from earlier interviews with police in which Nelson was uncertain if he had done the “God forgives” tattoo. He now says he is.

Prosecutors contend that Hernandez was in the front passenger seat of his Toyota 4Runner when he fired on de Abreu and Furtado. Hernandez’s lawyers allege that Alexander Bradley, the driver, shot the two men over a drug deal. Hernandez also has been charged with shooting Lloyd in February 2013 to silence him. Bradley, who is serving a five-year sentence in Connecticut for a shooting in a Hartford club in 2014, has been granted immunity by prosecutors in exchange for his testimony in the Hernandez trial.