Matthew Murray, the gunman who is believed to have shot and killed four people in Colorado, appeared to have acted out of revenge against Christians, police officials stated Tuesday.

Authorities believe Murray, 24, posted an anti-Christian message online on Sunday - the day of the shootings - in a language almost identical to the text of a manifesto written by one of the Columbine killers, Eric Harris.

"You Christians brought this on yourselves," Murray wrote, according to KUSA-TV in Denver. "Feel no remorse, no sense of shame, I don't care if I live or die in the shoot-out. All I want to do is kill and injure as many of you ... as I can especially Christians who are to blame for most of the problems in the world."

If the time on the posting is accurate, the diatribe was posted between the first shooting at 12:30am Sunday morning at a Youth With a Mission (YWAM) missionary training centre in Arvada and the second attack at 1:10pm that same day at New Life Church in Colorado Springs, according to KUSA.

The postings were removed after Sunday's killings from the unidentified website, which was designed for people who left evangelical religious groups. Other postings by Murray included lyrics by industrial rock band KMFDM.

"Our family cannot express the magnitude of our grief for the victims and families of this tragedy. On behalf of our family, and our son, we ask for forgiveness. We cannot understand why this has happened," Murray's uncle, Phil Abeyta, said in a statement as he fought back tears, according to The Associated Press.

Murray came from what neighbours have described as a "very, very religious" family. His 21-year-old brother, Christopher, is a student at Oral Roberts University in Tulsa, Oklahoma.

Murray, meanwhile, attended a home-based computer school and had worked at his computer for three to five hours a day for the past two years, investigators said.

In 2002, Murray had enrolled in a Discipleship Training School (DTS) at the YWAM Arvada training centre. DTS students partake in a 12-week lecture course followed by a 12-week field assignment, usually to another culture.

Murray neither completed the lecture phase nor participated in the field assignment. YWAM's USA International President John Dawson said programme directors felt "issues with Murray's health made it inappropriate" for him to participate.

Richard Werner, Murray's former roommate at the training centre, described Murray as a quiet and sarcastic person who made some bizarre comments.

At a YWAM Christmas festival in December 2002, Werner recalled Murray playing Marilyn Manson's "Sweet Dreams (are Made of This)" and Linkin Park's "One Step Closer" which included the lyrics "Cause I'm one step closer to the edge and I'm about to break".

"A lot of the kids really got scared about it," Werner told CNN. "We were just playing songs about Christmas, about God and friendship and then he came up with those songs."

Murray's parents were there and, with the centre's officials, they decided it would be best for him not to go out on a mission, according to CNN. Murray did not react angrily but was "always very calm", said Werner.

"After that, he got out of the meeting and he turned to a friend of mine and the only thing he said was like 'I have a message for everyone: just say that now you will see your star'," Werner told CNN.

Murray left the training centre and made no further visits, according to the YWAM statement.

He had, however, been sending hate mail to the centre, police said in court papers Monday.

Witnesses at YWAM's training centre said the gunman had entered and asked to spend the night at the dormitory but was turned down. He then opened fire, killing two young adults and wounding two others. They described him as a 20-year-old white male, wearing a dark jacket and skull cap.

About 12 hours later, witnesses at New Life Church in Colorado Springs described the gunman as wearing a dark trench coat and carrying a high-powered rifle.

He opened fire in the church parking lot, killing two teenage girls and wounding their father, and walked about 60 feet into the church before a church security guard - Jeanne Assam - shot him. He was struck multiple times by the officer, but a new report indicated Murray died after firing a single shot at himself, the El Paso County Coroner's Office concluded after an autopsy, according The Associated Press.

An 11:00am worship service had just ended about half an hour prior to the shooting and most worshippers had already left. Several hundred were still at the megachurch campus, according to New Life Senior Pastor Brady Boyd.

Police determined through investigations that "most likely the suspect in both shootings are one in the same".

Boyd said the shooting was a "random, senseless" act and that the gunman was unknown at New Life. The "common denominator", however, was Youth With a Mission - YWAM maintains a satellite office at the megachurch.

While news about the Arvada shooting had prompted New Life to beef up security that day, the 10,000-member church - the largest in the state - had always kept security volunteers at the church.

Boyd explained that in a culture that has become more open, churches have increasingly become targets of people who are "aggravated and upset".

Megachurches across the country are taking more precautions as violence against religious groups and churches is on the rise in the US. Some churches open their doors daily.

Earlier this year, megachurch pastor Bishop TD Jakes held his third annual S.T.O.P.P.E.D. - Security Training Offering Procedure Protection Education and Direction - conference, urging safety.

Authorities in Colorado Springs said the gunman at New Life had apparently intended to gun down many more victims before he was shot by Assam. He was carrying an assault rifle, two handguns and as many as 1,000 rounds of ammunition.

"God was with me," said Assam as she described her confrontation with the gunman. "God made me strong."