Police were searching a home in Englewood, Colorado, early Monday looking for clues in deadly attacks at religious institutions during the weekend.
The two shootings, the first at a Christian missionary center and the second at a Colorado Springs megachurch, left a gunman and four victims dead and six wounded, authorities said.
Arvada Police Chief Don Wick said there is reason to believe the shootings are related.
A black-clad gunman who attacked worshippers at New Life Church in Colorado Springs, initially killing one and wounding four, was killed by "a courageous security staff member," Colorado Springs Police Chief Richard Myers said.
One of the wounded died late Sunday, according to a Penrose-St. Francis Hospital spokeswoman Amy Sufak.
"The suspect was confronted by a security guard," Myers said at a news conference late Sunday. "She shot the suspect, and the suspect subsequently died at the scene."
Detectives in the Denver suburb of Arvada were working closely with their counterparts in Colorado Springs to determine whether the attack on New Life Church was related to the shootings that left two staffers dead and two wounded at a live-in training center for Christian missionaries about 12 hours earlier, Wick said.
At his late Sunday news conference, Myers said authorities from the two communities were cooperating in their investigations.
The identity of the man who opened fire at New Life Church had not been determined Sunday evening, Colorado Springs police spokesman Lt. Fletcher Howard said.
But witnesses' accounts of a black-clad gunman shared some elements with the description issued by Arvada police after the slayings in their town, about 80 miles north of Colorado Springs.
Brady Boyd, the senior pastor at New Life Church, said leaders of the nondenominational, evangelical congregation of more than 10,000 had beefed up security after the Arvada attacks.
"Many lives were saved because of the quick action of some committed volunteers at our church," Boyd said. He said about 7,000 people were at the church, which had just completed a late-morning service, when the shooting took place shortly after 1 p.m.
In the earlier shooting, Arvada police said a dark-jacketed man with a beard, glasses and skullcap entered Youth With a Mission about 12:30 a.m. Sunday and opened fire after a dispute with a staff member about whether he would be allowed to stay the night there.
"The staff member contacted other staff members to help remove the suspect from the residence," Wick said. "Upon leaving the facility, the suspect turned and fired a handgun numerous times, striking four victims."
Youth With a Mission staff members Tiffany Johnson, 26, and Philip Crouse, 24, died in surgery after the shooting, the group's co-founder, Peter Warren, said.
He said they were cleaning up from a Saturday night Christmas banquet when the attack occurred.
Another staff member, 24-year-old Dan Griebenow, was in critical condition with a bullet in his neck, and a second survivor, Charlie Blanch, was shot in the legs, the group said on its Web site.
Investigators tried to track the gunman through fresh snow with the help of dogs, but lost his trail in a heavily walked area, Deputy Chief Gary Creager said.
Youth With a Mission was founded in 1960, and operates in more than 1,000 locations in 149 countries, according to its Web site. The Arvada center is home to dozens of people from around the world training as Christian missionaries.
"These kids were like our kids, you know?" Warren told CNN affiliate KUSA. "It's just such a tragedy."
In Colorado Springs, New Life member Ashley Gibbs said she heard five shots in quick succession as she and her boyfriend headed for their car. A short time later, she heard several more shots -- and saw one hit the snow about 100 feet away from her car.
"I saw the guy for just a second," she said. "He was wearing a big, black trenchcoat and carrying a big gun."
Gibbs said she hadn't heard about the shootings in Arvada, and said she was "completely shocked" by the sound of gunfire.
"I never thought that anything like this would ever happen," she said. "That's why when I heard the gunshots, I didn't think they were gunshots."
Gibbs description of the man -- in his early to mid-20s, dressed in a dark coat -- sounded similar to the description of the man behind the Arvada killings.
But she could not recall any facial hair, as survivors of the attack on the missionary center described. And she said she did not know what kind of weapon he carried, but said it was "a big one -- you had to hold it with two hands."
When it was over, Myers said, the gunman and one church member were dead on New Life's grounds.
The nondenominational New Life Church was founded by the Rev. Ted Haggard, an evangelical Christian leader ousted in 2006 after allegations that he had been a client of a male prostitute from whom he had purchased drugs.