COLORADO SPRINGS, Colorado (CNN) -- "It seemed like it was me, the gunman, and God," said Jeanne Assam, describing her feelings as she confronted a man who charged into her Colorado Springs church Sunday firing a weapon.
Assam, a church security guard with law enforcement experience, fired her own weapon at the invader and stopped his attack, police say.
Police on Monday identified the gunman as Matthew Murray and said he was also responsible for an attack earlier Sunday at a missionary center some 80 miles away.
The two incidents left four people dead, in addition to the gunman, and five wounded.
Police said forensic evidence showed a positive match between a handgun found at the church and shell casings found at the missionary center in Arvada, a suburb of Denver.
Officials from the Colorado Springs and Arvada police departments appeared at a joint news conference to brief reporters on what they had pieced together about Sunday's events.
The first incident took place Sunday at about 12:30 a.m. at Youth With a Mission in Arvada.
Police said a man with a beard, wearing a dark jacket, glasses, and skullcap, entered the facility, got into a dispute with a staff member about whether he could stay there for the night, and then opened fire.
Two staff members, Tiffany Johnson, 26, and Philip Crouse, 24, died later Sunday from their injuries, Youth With a Mission co-founder Peter Warren said. Two other staff members were injured, and one was in critical condition.
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 of Arvada police said.
About 12 hours later, police say, Murray showed up at New Life Church as a service was letting out.
Police said Monday he had an assault rifle and two handguns, and may have had as many as 1,000 rounds of ammunition.
He fired on a family who were in or near their car. Two sisters, identified by police as Rachael Works, 16, and Stephanie Works, 18, were killed, and their father, David Works, 51, was also shot and is hospitalized in fair condition with two gunshot wounds.
Murray then entered the church, police say, where Assam was one of several volunteer security guards on duty.
"I saw him coming through the doors," she told reporters on Monday. "I took cover, and I waited for him to get closer, and I came out of cover and identified myself, and engaged him, and took him down. And that's pretty much it." Watch Assam tell how she stopped Murray »
Police said they were still investigating whether Assam's weapon killed Murray, or whether he might have died of a self-inflicted gunshot.
Assam extended her sympathy to the families of the victims "and of the gunman -- and I mean that very sincerely."
Phil Abeyta, who identified himself as Murray's uncle, appeared at another news conference and read a statement from the family asking for forgiveness.
"Our family cannot express the magnitude of our grief for the victims and families of this tragedy," he said. "On behalf of our family and our son, we ask for forgiveness. We cannot understand why this has happened."
Abeyta appeared with spokesmen from the Youth With A Mission center, who confirmed that Murray had been part of a training program five years ago.
Peter Warren, director of Youth With A Mission, said Murray did not go on the mission he was training for in 2002 because managers thought that "issues relating to his health made it unsafe for him to do so."
But a man who served at the center with Murray told CNN Monday that Murray was kicked out of the mission program for strange behavior. Watch more about Matthew Murray's background »
A source -- a long-time member of New Life Church -- said Murray had a falling out with Youth With A Mission after working with the organization a couple of years ago. The source said Murray sent antagonistic and threatening correspondence afterward.
Earlier Monday, Boyd said the gunman was unknown to parishioners there.
"He simply showed up on our property yesterday with a gun, with the intention of hurting people, and he did," Boyd said.
Boyd said the megachurch instituted security precautions after the shootings at the Denver area mission center.
Boyd said Assam was a hero in preventing further bloodshed, rushing to confront the gunman just inside the church. "She probably saved over a hundred lives," Boyd said. Watch pastor tell how guard saved lives »
"I give the credit to God. And I say that very humbly. God was with me and the whole time I was behind cover -- this has got to be God, because of the firepower that [the gunman] had vs. what I had," Assam said.
"I did not run away and I didn't think for a minute to run away, I just knew that I was given the assignment to end this before it got too much worse. I just prayed for the Holy Spirit to guide me."
She told reporters she had not slept since the shooting, "as I'm sure you can tell."
She said she was on the third day of a three-day regimen of fasting and prayer, wanting to know God's will for what to do with her life, when the shooting took place.
"I was weak, and where I was weak, God made me strong," she said. "He filled me and he guided me and protected me and many other people. And I'm honored that God chose me."
Assam was one of about a dozen volunteer security guards at the church, half of whom are armed, Boyd said. The guards are licensed, trained and screened, and are church members, not "mercenaries," he said. E-mail to a friend
CNN's Sean Callebs contributed to this report.