YUMA, Ariz. >> An Arizona prisoner was convicted today of the gruesome killings of his best friend, the man’s girlfriend and her four young children 12 years ago in what prosecutors called a crime motivated by money.

A jury found Preston Strong, 50, guilty of six counts of first-degree murder following a monthslong trial. Prosecutors are seeking the death penalty, and the jury will begin considering Strong’s eligibility Friday.

Prosecutors said Strong, who is already serving a life sentence for a separate slaying, spent hours killing 35-year-old Luis Rios, 29-year-old Adrienne Heredia and her four children, ages 6 to 13, in Yuma in 2005.

They said he suffocated Heredia and three of her children and fatally shot her youngest child and Rios. Police said the two oldest children were bound and strangled. A 9 year-old girl also had been strangled, and the 6-year-old was bound by electric cords, police said.

“This crime required information and access to these victims,” prosecutor Karolyn Kaczorowski said during closing statements. “It required motives; it required a great deal of opportunity because, as you’ve seen and what we will discuss, is these murders happened over at least six hours’ worth of time. That’s a huge chunk of opportunity that’s needed.”

Prosecutors said Strong and Rios had been arguing over money and how much time he had been spending with Rios and her children. Prosecutor John Tate said Rios had given Strong thousands of dollars over the years but decided to cut him off at some point before the killings, leading to a fight between the friends.

Prosecutors also said fingerprint evidence was found on bags that were discovered near some of the victims while DNA evidence was on a nightstand and on the steering wheel of Rios’ car.

The defense said the evidence was flimsy and that Strong’s DNA could have gotten on the family’s items because he spent a lot of time with them. They cast doubt on two eyewitnesses who told police they had seen a Hispanic man leaving the crime scene. Strong is black.

Strong’s attorneys said an ex-girlfriend mentioned she was with Strong during a time when police say he had to have been committing the killings.

“It is a leap to say that because he had prints on a bag that he’s guilty of murder. That’s a leap,” attorney Ray Hanna said.

Strong is in prison for life in the 2007 killing of Satinder Gill, a Yuma physician who was strangled and bludgeoned in his home. Prosecutors said a large amount of money was stolen.