'How can this happen here?': Police seek motive after gunman kills 6, then self, at Colorado birthday party

Police in Colorado were searching for a motive Monday after a gunman opened fire at a birthday party over the weekend, killing six people before killing himself.

The shooting at a Colorado Springs, Colorado, mobile home park early Sunday is the 13th mass killing this year, according to database compiled by USA TODAY, Northeastern University and The Associated Press.

Colorado Springs police responded to reports of shots fired shortly after midnight Sunday at the Canterbury Mobile Home Park and found six people dead. A seventh was taken to the hospital where he later died.

"The suspect, a boyfriend of one of the female victims, drove to the residence, walked inside and began shooting people at the party before taking his own life. This horrific act has resulted in the death of six adult victims," Colorado Springs Police Department said in a statement.

The group was gathered inside the residence for a birthday party, and friends, family and children all inside, police said. No children were injured in the shooting, and all were with relatives Sunday.

Police said on Twitter that they would release the victims' names in a Tuesday news conference.

A mobile crime lab was parked early Monday outside the Colorado Springs home, which was cordoned off by yellow police tape as officers guarded the scene. By midafternoon the tape was gone; a sign stood in front of the home advertising “Grief Support, 10 a.m. to 6 p.m.," at the mobile home park office.

A small crowd of mourners arrived to pay their respects, leaving bouquets of yellow roses and devotional candles on a small table they set up in front of the home. They silently gazed, then left without comment under a dark, gray sky and blustering winds. Someone closed a partially open window in the home from the outside.

Freddy Marquez told the Denver Post he was at the party before the shooting with his wife and children, and the victims were all of the same extended family. The party was to celebrate his wife's and her brother's birthdays, but Marquez said his family left around 10 p.m. because his wife had to work the next day.

'Unspeakable acts': Gunman kills 6, then self, at birthday party in Colorado Springs, Colorado, police say

“It was all family, so everything was fine,” he told the newspaper. When his wife woke up the next morning, she saw numerous missed calls. Marquez's wife's mother, two brothers and three extended family members died. “It’s just crazy; it’s not what we expected on Mother’s Day,” he told the Post. “I’m at a loss for words.”

Neighbor Yenifer Reyes told the newspaper she thought the gunshots she heard were a thunderstorm but, "then I started hearing sirens."

Gladis Bustos, who lives three homes away, tearfully recalled the home’s owner, whom she identified as Joana, as a warmhearted, hardworking person who always took the time to say hello to her neighbors, ask how they were doing, and brag about her children.

“She was an incredibly pleasant woman, very beautiful, happy all the time,” Bustos told AP. “She loved to chat. And she was very proud of her family.”

The gunfire that startled Bustos from her sleep early Sunday had forever changed what she described as a safe and welcoming neighborhood. “We’re all in shock," she said. “How can this happen here? This is all so painful, so devastating, so overwhelming.”

Authorities have not yet released the identities of the victims, and police said they are still investigating a motive for the shooting. Marquez told the Denver Post he didn't know the suspected shooter well and the man was not at the party earlier.

"This is something you hope never happens in your own community, in the place that you call home. When these types of unspeakable acts happen, there is nothing that can be done to fully rebuild what was lost or replace those who are no longer with us," Colorado Springs Police Chief Vince Niski said in a statement.

Colorado Gov. Jared Polis said on Twitter that the shooting was "devastating, especially as many of us are spending the day celebrating the women in our lives who have made us the people we are today.”

According to the Centers for Disease Control and Prevention, crime report data indicates about 1 in 5 homicide victims in the U.S. are killed by an intimate partner. Over half of female homicide victims are killed by a current of former male partner, the CDC says.

The massacre is the second mass killing in Colorado this year, according to the USA TODAY, Northeastern University and The Associated Press database, which defines mass killing as those in which four or more people were killed, not including the assailant.

Columbine, Aurora, Boulder: Colorado has the sixth-highest rate of public mass killings

In March, 10 people were killed in Boulder when a gunman opened fire at a grocery store. The shooting occurred less than a week after rampage at Atlanta-area spas left eight people dead, mostly women of Asian descent.

The shooting this weekend is the deadliest in the U.S. since a former employee at an Indianapolis FedEx facility killed eight people before dying by suicide.

Three of the 13 mass killings this year have occurred in Indiana, said James Alan Fox, professor of criminology, law and public policy at Northeastern University. Seven of the 13 were in residential settings; an eighth was partially in a residence, he said.

According to the Gun Violence Archive, another database that defines mass shooting as four or more shot or killed, not including the shooter, May has already seen 24 mass shootings.

Contributing: Elinor Aspegren, USA TODAY; The Associated Press

This article originally appeared on USA TODAY: Colorado Springs shooting: Police seek motive in birthday party attack