Frantic 911 calls from the US movie theatre where a massacre was under way were played in court on Tuesday, on the second day of hearings to decide if the alleged gunman should stand trial.
The court also heard how suspected killer James Holmes purchased more than 6000 rounds of ammunition in the months before last July's shooting, and rigged up elaborate booby-traps involving gasoline and napalm at his apartment.
In the first of 41 harrowing emergency calls made in a 10-minute period in Colorado, 30 loud booms could be heard in less than half-a-minute, making it difficult for the 911 dispatcher to distinguish what was being said.
In another, six minutes after the first call logged at 12:38am, 14-year-old Kaylan Bailey told the emergency operator her two cousins had been shot, and that one of them did not appear to be breathing.
"We need to start CPR (cardio-pulmonary resuscitation) on your cousin who's not breathing," said the operator.
"I can't hear you," responded Bailey, as chaos unfolded in the theatre in Aurora, outside Denver, where Holmes had thrown at least one smoke-bomb type device into the auditorium before opening fire in the early morning.
The emotionally fraught hearings opened on Monday with police describing harrowing scenes as they responded to the theatre shootings, where 25-year-old Holmes is accused of opening fire after on 20 July, wearing body armour and a mask.
A number of officers choked back tears as they testified about the night, in which 12 people were killed and dozens more injured at a packed midnight screening of the Batman movie The Dark Knight Rises.
The Aurora massacre revived the perennial US debate over gun control — an issue re-ignited even more intensely by last month's shooting of 20 young children at a Connecticut elementary school.
Tuesday's hearing heard testimony from federal firearms supervisor Steven Beggs that Holmes made at least 16 purchases from May to July 2012 including four firearms, incendiary devices and almost 6300 rounds of ammunition.
Holmes's lawyers could argue that he is mentally unfit to stand trial. Defence attorney Tamara Brady asked Beggs: "Is there any process in Colorado to screen out purchases by a severely mentally ill person?"
"No," replied Beggs, who detailed the purchases made online and in person by Holmes.
FBI bomb technician Garrett Gumbinner said Holmes told investigators after being arrested that he had booby-trapped his apartment, in an attempt to draw emergency responders away from the theatre shooting site.
Officers found a trip wire at the door of his home, rigged to set off flames and sparks that would catch the gasoline-soaked carpet and go on fire.
There were six-inch fireworks shells filled with improvised thermite, a hot-burning explosive.
"You can't put it out with water," Gumbinner told judge William Sylvester. There were also three containers of improvised napalm, 11 bottles of gasoline and other chemicals intended to act as fire accelerants, he said.
There were three systems in the apartment intended to initiate the explosive devices, including one linked to a trash bag near the apartment dumpster by a remote control.
It used a remote vehicle to attract someone to the bag to play with the car, triggering an explosive controller in the apartment, Gumbinnger said Holmes told the officers.
Prosecutors are trying to build up their case that the shootings were a premeditated act of mass murder, and identifying Holmes as the lone shooter.
Supporting that strategy, Aurora police officer Tom Welton testified that Holmes made postings on two dating websites earlier in July, asking "Will you visit me in prison?"
Welton also identified two people in the theatre who say they saw a single person wearing body armour enter the theatre about 18 minutes after the movie had begun.
The first was Tara Bahl, seated in the second row of the auditorium.
"She said he threw a smoking projectile in the seats behind her and began shooting," Welton said.