AURORA, Colo. — The Colorado shooting suspect planned the rampage that killed 12 midnight moviegoers with “calculation and deliberation,” police said Saturday, receiving deliveries for months that authorities believe armed him for battle and were used to rig his apartment with dozens of bombs.

Authorities on Saturday were still working to clear dangerous explosive materials from inside James Holmes’ suburban Denver apartment a day after police said he opened fire and set off gas canisters in a suburban theater minutes into the premiere of the Batman film “The Dark Knight Rises.” The attack left 12 dead and 58 injured.

His apartment was rigged with jars of liquids, explosives and chemicals that were booby trapped to kill “whoever entered it,” Aurora Police Chief Dan Oates said, noting it would have likely been one of his officers.

“You think we’re angry? We sure as hell are angry,” Oates said.

Authorities wouldn’t discuss a motive for one of the deadliest mass shootings in recent U.S. history, as makeshift memorials for the victims sprang up and relatives began to publicly mourn their loved ones. Holmes had recently withdrawn from a competitive graduate program in neuroscience; neighbors and former classmates in California have said he was a smart loner who said little.

But he had apparently prepared the attack at the Aurora theater well in advance, receiving multiple deliveries by mail for four months and buying thousands rounds of ammunition on the Internet, Oates said.

“He had a high volume of deliveries to both his work and home address. We think this explains how he got his hands on the magazine, ammunition,” he said, as well as the rigged explosives in his apartment.

“What we’re seeing here is evidence of some calculation and deliberation,” Oates added.

Federal authorities detonated one small explosive and disarmed others inside Holmes’ apartment after sending in a robot to take down a trip wire, FBI Special agent James Yacone said. Bomb technicians then neutralized what he called a “hyperbolic mixture” and an improvised explosive device containing an unknown substance. There also were multiple containers of accelerants, he said.

“It was an extremely dangerous environment,” Yacone said, saying anyone who walked in would have sustained “significant injuries” or been killed.

Holmes, 24, was in a county detention facility Saturday, held without bond on suspicion of multiple counts of first-degree murder. He was set for an initial hearing on Monday and had been appointed a public defender, authorities said.

Stories of the dead began to emerge, including a 6-year-old girl and a man who died on his 27th birthday and a day before his anniversary. Families grieved and waited at hospitals, which reported at least seven wounded still in critical condition Saturday and others with injuries that were likely permanent.

Veronica Moser, 6, had gone to the movies with her mother, who was drifting in and out of consciousness in a hospital intensive care unit, who had gunshot wounds in her throat and abdomen.

“Nobody can tell her about it,” Annie Dalton said of her niece, Ashley Moser. “She is in critical condition, but all she’s asking about is her daughter.”

Veronica had just started swimming lessons on Tuesday, Dalton said.

“She was excited about life as she should be. She’s a 6-year-old girl,” her great-aunt said.

Another victim, 27-year-old Matt McQuinn, was killed after diving in front of his girlfriend and her older brother to shield them from the gunfire, said his family’s attorney, Rob Scott of Dayton, Ohio.

Alex Sullivan had planned a weekend of fun to ring in his 27th birthday with friends at the special midnight showing of “The Dark Knight Rises” and then celebrate his first wedding anniversary on Sunday.

“He was a very, very good young man,” said Sullivan’s uncle, Joe Loewenguth. “He always had a smile, always made you laugh. He had a little bit of comic in him.”

Oates said Holmes used a military-style semi-automatic rifle, a shotgun and a pistol to open fire on the unsuspecting theater-goers. He had bought the weapons at local gun stores within the past two months. He recently purchased 6,000 rounds of ammunition over the Internet, the chief said.

Holmes also bought an urban assault vest, two magazine holders and a knife for just over $300 on July 2 from an online supplier of tactical gear for police and military personnel, according to the company.

Chad Weinman, CEO of TacticalGear.com, said his company processes thousands of orders each day, and there was nothing unusual in the one that Holmes placed. While his company often receives orders from military units and law enforcement organizations, it is not out of the ordinary for individual police officers or soldiers to place orders, he said.

“Everything Mr. Holmes purchased on July 2 is commercially available,” Weinman said, adding he was “appalled” that the material was sold to Holmes before the shooting.

It wasn’t known why the suspect chose a movie theater to stage the assault, or whether he intended some twisted, symbolic link to the film’s violent scenes.

The Batman movie, the last in the trilogy starring Christian Bale, opened worldwide Friday with midnight showings in the U.S. The plot has the villain Bane facing Bale’s Caped Crusader with a nuclear weapon that could destroy all of fictional Gotham.

“The Dark Knight Rises” earned $30.6 million in Friday morning midnight screenings, and, according to industry estimates, roughly $75-77 million on the day. That put it on track for a weekend total of around $165 million, which would be the second-highest opening weekend ever, following “The Avengers.”

Warner Bros. has announced it would forgo the usual revenue reports until Monday out of respect for the victims. Sony, Disney and Universal also said they would delay reporting box office receipts until Monday, a day later than the routine Sunday releases for Hollywood.

After buying a ticket to the movie, Holmes went into the theater and propped open an exit door several minutes into the film, a federal law enforcement official said. The suspect then returned in protective gear and with high-powered weapons and opened fire, shooting scores of people and picking off victims who tried to flee, officials said.

The shooting was the worst in the U.S. since the Nov. 5, 2009, attack at Fort Hood, Texas. An Army psychiatrist was charged with killing 13 soldiers and civilians and wounding more than two dozen others. It was the deadliest in Colorado since the 1999 attack at Columbine High School, where two students killed 12 classmates and a teacher and wounded 26 others before killing themselves.

After excelling at the University of California-Riverside, Holmes was in the process of withdrawing from a neuroscience Ph.D. program at the University of Colorado-Denver for unknown reasons.

First-year students must take a three-part exam at the end of the academic year to move on in the program, University spokeswoman Jacque Montgomery said Saturday. Montgomery did not know whether Holmes had taken the exam.

As part of the program, Holmes had been listed as making a presentation in May about Micro DNA Biomarkers in a class named “Biological Basis of Psychiatric and Neurological Disorders.”

Mary Muscari, a criminology professor at Regis University in Denver who studies mass killings, said she was not surprised Holmes was studying neuroscience and mental disorders.

“It could be he was interested in that because he knows there’s something different in him,” said Muscari.

She said that several mass murderers are young men in late adolescent or early adulthood. “We’re talking about guys when they’re at that ago when they’re testosterone charged, their brains start developing, and it’s also when schizophrenia kicks in.”

Those who knew Holmes described him as a shy, intelligent person raised in California by parents who were active in their well-to-do suburban San Diego neighborhood. Holmes played soccer at Westview High School and ran cross-country before going to college.

Police said they would begin collecting the personal items left by panicked moviegoers in days and would move out of the theater by midweek. Shaken law enforcement officials urged residents to not stay home.

“I just don’t want the shameless and senseless act of one man to make this difficult for families to move on,” Aurora Fire Chief Mike Garcia said. “Go out. See a movie. Go out into your city. Don’t be afraid.”