ALBUQUERQUE, N.M. (AP) — A lawyer for an Afghan refugee accused in the Albuquerque slayings of three Muslim entered a not guilty plea Friday on her client’s behalf as the community continues its struggle to understand the motives behind the killings.

Muhammad Syed, 51, appeared remotely for the court hearing and will remain held without bond pending trial. He is charged with three counts of murder and tampering with evidence, and police have identified him as the suspect in the killing of a fourth Muslim man.

Syed, who has been in the U.S. with his family for several years, previously denied involvement in the killings when authorities detained him earlier this month.

Authorities have not disclosed a motive for the killings, but prosecutors have described Syed as having a violent history. His public defenders have argued that previous allegations of domestic violence against Syed never resulted in convictions.

Authorities have said they have linked bullet casings found at two of the crime scenes with casings found in Syed’s vehicle and with guns found at his home and in his vehicle.

Syed was arrested Aug. 8 more than 100 miles (160 kilometers) from his Albuquerque home after tips led investigators to the Syed family. He told authorities he was on his way to Texas to find a new home for his family, saying he was concerned about the ambush-style killings.

Syed has been charged with these killings:

— Aftab Hussein, 41, was slain July 26 after parking his car in his usual spot near his home.

— Muhammad Afzaal Hussain, a 27-year-old urban planner who had worked on the campaign of a New Mexico congresswoman, was gunned down Aug. 1 while taking his evening walk.

— Naeem Hussain was shot Aug. 5 as he sat in his vehicle outside a refugee resettlement agency on the city’s south side following funeral services for two of the other shooting victims. Shots were fired at Hussain’s SUV, striking him in the head and the arm.

Syed is the primary suspect — but hasn’t been charged — in last November’s slaying of Muhammad Zahir Ahmadi, a 62-year-old Afghan immigrant who was fatally shot in the head behind the market he owned.

Muhammad Imtiaz Hussain, the older brother of Muhammad Afzaal Hussain, said in an interview Friday that his family is heartbroken and frustrated because they have no idea why the young man from Pakistan would have been targeted or how he would have crossed paths with Syed.

The two men were from different Islamic backgrounds. Syed speaks Pashto and no English. Muhammad Afzaal Hussain, who was the son of an elementary school teacher, studied law and human resource management at the University of Punjab before coming to the U.S. in 2017.

“These questions are rattling in my mind,” the victim’s brother said. “If you punish him (Syed) for 10 years, 20, 30, 1,000 or a million years, how would I satisfy myself if I don’t know why he killed my brother? What happened to him? For me, for justice, we need to know why.”

Muhammad Afzaal Hussain was working as the city of Española’s planning and land use director after receiving a master’s degree from the University of New Mexico. During his time at UNM, he became a student leader and an advocate for the international community.

His colleagues and those in political circles described him as having a bright future. His brother said his ultimate goal was to open a school in their hometown in Pakistan so other children could be afforded a quality education.

Muhammad Imtiaz Hussain said that mission will continue.

“My brother died but we will aim to make more brothers and sisters like him who can inspire people, who work for the benefit of humanity, who help others, who raise their voice for others,” he said.