COLORADO: The suspect jailed on 10 counts of murder in this week's mass shooting in Colorado has been moved to another detention center because of "safety concerns and threats," and he faces additional attempted-murder charges, officials said on Friday.

Boulder County District Attorney Michael Dougherty and Boulder Police Chief Maris Herold told reporters they were still trying to learn what drove Ahmad Al Aliwi Alissa to open fire on Monday at the King Soopers supermarket in Boulder, some 28 miles (45 km) northwest of Denver.

Alissa, 21, surrendered to law enforcement officials at the crime scene after he was wounded in an exchange of gunfire with police, authorities said.

Ten people were killed in the rampage, including a police officer. It was the second mass shooting in less than a week in the United States, after a gunman fatally shot eight people at three Atlanta-area day spas on March 16.

"Like the rest of the community, we want to know why. Why that King Soopers, why Boulder, why Monday, and unfortunately we don’t have those answers," Herold said at a news briefing. "It will be something haunting for all of us until we figure that out."

Asked whether authorities suspected any link to international terrorism, Dougherty said investigators were looking into all aspects of Alissa's background.

Alissa, who made his first court appearance on Thursday, has been charged with 10 counts of first-degree murder and a single count of attempted murder, stemming from gunshots he allegedly fired at a second police officer. Dougherty said the suspect would face further attempted-murder charges in the coming weeks.

The suspect, being held without bail, has been transferred to another unspecified lockup outside Boulder County "due to safety concerns and threats that our jail staff became aware of," sheriff's spokeswoman Carrie Haverfield told Reuters. She did not elaborate.

The public defender's office, assigned to represent Alissa, said it does not comment on pending cases as a matter of policy.

The suspect purchased a Ruger AR-556 pistol, a weapon that resembles a semi-automatic rifle, six days before the attack. He did not use a 9mm handgun also in his possession during Monday's attack, Dougherty said.

The store where Alissa bought the Ruger said the suspect passed a background check before making the purchase legally.

"We are absolutely shocked by what happened and our hearts are broken for the victims and families that are left behind," said John Mark Eagleton, owner of the Eagles Nest Armory in Arvada, the Denver suburb where Alissa lived.

Dougherty said federal agents were investigating the two guns recovered from the shooting and other weapons linked to the suspect, while a large forensics team continued to process the sprawling crime scene inside the supermarket.

Alissa's brother has said the suspect suffered from paranoia, and his lawyer on Thursday asked in court that he be given a mental health examination. Alissa pleaded guilty to a third-degree assault in 2018 for punching a high school classmate without warning the previous year.

The Boulder and Atlanta attacks have reignited a national debate over gun rights protected by the U.S. Constitution's Second Amendment, and prompted President Joe Biden to call for federal action aimed at curbing gun violence. A bill intended to impose stricter background checks and ban some types of semi-automatic rifles has stalled amid Republican opposition.

Colorado has enacted laws that require background checks for gun purchases, limit the ammunition magazine capacity and enable courts to temporarily seize the gun of someone deemed dangerous. Seven other states and the District of Columbia have banned certain types of semi-automatic rifles.

The United States banned them nationwide in 1994, but a more conservative U.S. Congress allowed the law to expire in 2004. Federal gun control legislation has mostly failed ever since, even after a gunman killed 20 young school children and six adults in a Connecticut school just before Christmas 2012.