The 21-year-old suspect in this week’s Colorado mass shooting legally purchased the semi-automatic weapon used in the attack, and he will face additional charges for attempted murder, officials in Boulder County said on Friday.

Ahmad Al Aliwi Alissa has been charged with 10 counts of murder and an attempted murder charge stemming from the rampage that killed 10 people at King Soopers supermarket in Boulder, some 28 miles (45 km) northwest of Denver.=

Boulder County District Attorney Michael Dougherty and Boulder Police Chief Maris Herold told a briefing that Alissa, 21, purchased a Ruger AR-556 pistol, a weapon that resembles a semi-automatic rifle, legally six days before the attack.

Alissa did not use a handgun also in his possession, they said.

Both said Alissa’s motive remained unclear.

“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.

Alissa made his first court appearance on Thursday. He is being held without bail and is due to undergo a mental health examination.

Dougherty said additional first-degree attempted murder charges would be filed against Alissa in the coming weeks.

Authorities have not disclosed a possible motive for the killings.

The bloodshed at King Soopers was the nation’s second mass shooting in less than a week, after a gunman fatally shot eight people at three Atlanta-area day spas on March 16.

The two attacks have reignited a national debate over gun rights and prompted President Joe Biden to call for new legislation from Congress and consider executive actions to try to stop gun violence. A bill intended to impose stricter background checks and ban certain types of semi-automatic rifles has stalled amid Republican opposition.

Reporting by Nathan Layne in Wilton, Connecticut; editing by Jonathan Oatis / REUTERS