Boulder mass shooting: 9 civilians killed before officer, slain after, arrived

The nine civilian victims gunned down at a Boulder, Colorado, grocery store last month were all killed before police officer Eric Talley and other officers "bravely charged in" the store, Boulder County District Attorney Michael Dougherty said Thursday.

Less than 30 seconds after Talley -- the final victim -- was shot and killed, a "second wave of officers" headed into the store, Dougherty said at a news conference.

Prosecutors have added 43 additional charges against Ahmad Al Aliwi Alissa, who was taken into custody after the March 22 shooting that killed 10 at the King Soopers grocery store, Dougherty said.

Alissa, 21, was initially charged with 10 counts of first-degree murder as well as one count of attempted murder for allegedly shooting at a police officer who was not hurt.

The amended criminal complaint list numerous counts of attempted murder for the citizens and police officers who were in the store and responded to the shooting.

At least 115 people were in the store and at least 25 others were in the parking lot when the shooting began, Dougherty said.

New charges also include assault and for the illegal large capacity magazines Alissa was allegedly carrying.

The weapon allegedly used in the massacre was a semi-automatic Ruger AR-556 pistol that was bought legally in Colorado, police said.

A motive has not been determined, Dougherty said Thursday. He said authorities are still working to uncover one, but added, "you don't need motive to prove that someone acted with intent."

Alissa has not entered a plea. He is next due in court on May 25 for a status conference, Dougherty said.

ABC News' Alex Stone contributed to this report.