Lawmakers' reactions expressed a range of emotions, including sympathy for the families, horror and grief. One member, though, wondered why no one in the theater was carrying a gun himself or herself to stop the killer.
Colorado Senator Mark Udall said in a statement: "I am deeply saddened and outraged by these senseless and tragic shootings," Udall said. "My thoughts go out to the families and friends of those killed, and I am praying for a speedy recovery for those who were wounded. My staff and I are in close contact with officials on the ground and will offer any federal resources needed to investigate this terrible crime and bring those responsible to justice."
House Minority Leader Nancy Pelosi called the shootings a "senseless act of violence that took the lives of innocent people."
"As local law enforcement, first responders, and medical professionals work tirelessly in the aftermath, the federal government will be a partner," Pelosi said, in a statement. "The thoughts and prayers of all Members of Congress are with Coloradans this morning as they grapple with the random, senseless act of violence that took the lives of innocent people," she said.
Colorado Rep. Diana DeGette called the shooting "inexplicable" and a "horrific act of violence."
"We are all profoundly saddened by this senseless attack. My family and I join with all Coloradoans and those across the country today in praying for the victims and their families and loved ones," she said in a statement. "While we do not yet know the identities of the victims, in the coming days and weeks I know that all of us in the Denver-area will offer our support and assistance to those impacted by this shocking tragedy."
House Speaker John Boehner said that, "Confronted with incomprehensible evil, Americans pull together and embrace our national family more tightly."
"I join President Obama, and every American, in sending my thoughts and prayers to the victims of this awful tragedy. We will all stand with them, as one nation, in the days ahead," said Boehner, in a statement.
Senate Majority Leader Harry Reid said, "Innocent people suffered a heartbreaking loss, but the victims and their families are not alone. Today, Americans take time to reflect on the value of life and the things that are most important to us, and mourn for those who lost what is most important to them."
Senate Minority Leader Mitch McConnell also weighed in:
"This senseless massacre of so many innocent people gathered with friends and family in a movie theater reminds us not only of the great evil that exists in the hearts of some, but of the great and precious gift of life," he said in a statement.
Texas Republican Rep. Louie Gohmert's reaction differed from other members' :
"It does make me wonder, with all those people in the theater, was there nobody that was carrying a gun that could have stopped this guy more quickly?"
House Democratic Whip Steny Hoyer called on Americans to come together in a statement:
"I join in mourning the tragic loss of life in Colorado this morning. The families of the victims, the many injured, and all those in Aurora are in my thoughts and in my prayers. It is in times like these that Americans have always rallied together as one community and one family, and we do so again today," Hoyer said.
Ohio Republican Sen. Rob Portman said:
"Jane and I join all Americans in expressing our condolences to the families and friends of those whose lives were cut short by the senseless act of violence in Colorado last night. They are in our prayers this morning, as are those victims who were injured and a community now in grief."
New Jersey Democratic Sen. Frank Lautenberg said, "The shooting in Aurora is a horrific act of violence, and our thoughts go out to the innocent victims and their families. Our hearts are filled with sadness for the 12 people killed and the dozens wounded in this senseless act," Lautenberg said. "We have to face the reality that these types of tragedies will continue to occur unless we do something about our nation's lax gun laws."
This post has been updated. Originally published at 9:26 a.m. Billy House and Michael Catalini contributed to this report.