Denver, Mar 27 (IANS): The weapon used by the suspect, a Syrian immigrant, to carry out the mass shooting at a grocery store in the US state of Colorado earlier this week was purchased legally, authorities said.

Addressing a press conference on Friday, Boulder Police Chief Maris Herold said that the weapon allegedly used by 21-year-old Ahmad Al Aliwi Alissa, was a semi-automatic Ruger AR-556 pistol, which was purchased legally from a shop in nearby city of Arvada, reports Xinhua news agency.

Ten people, including one police officer, were killed in the carnage on Monday at the King Soopers supermarket in Boulder city.

Alissa reportedly purchased the Ruger AR-556, a short-barreled AR-15 rifle, days before the mass shooting, according to Herold.

The store owner who sold the gun said that the suspect had passed a background check required by state authorities, local newspaper The Denver Post reported.

Officials said the suspect also had a second weapon, a 9 mm handgun, with him but he did not use it in Monday's shooting.

Alissa, a resident in Arvada, is charged with 10 counts of first-degree murder and one count of attempted first-degree murder over shooting at police.

Officials said they are planning to file additional charges of attempted murder against the suspect in the next couple of weeks.

Officials said that the motive of the shooting is still unclear.

Herold noted that 26 agencies have been working 24 hours a day to investigate and "like the rest of the community, we, too want to know why. Why King Soopers, why Boulder, why Monday and unfortunately we don't yet have those answers".

Before this incident, tragic mass shootings has occurred in the state.

On July 20, 2012, a shooter set off tear gas grenades and shot inside a movie theatre in Aurora, killing 12 people and injuring 70 others.

It was the deadliest shooting in Colorado since the Columbine High School massacre in 1999, in which two young killers murdered 12 students, a teacher and injured 21 others.