DENVER - Three people were shot to death and six were wounded in Colorado on Sunday in two church-related shootings in the U.S. Christian heartland.

A gunman -- described by an eyewitness as dressed in black, wearing combat boots and holding an assault rifle and at least one handgun -- wounded four people when he opened fire in the parking lot of the vast New Life evangelical church in Colorado Springs, Colorado, after Sunday services, police said.

A New Life church security guard shot and killed the gunman before police arrived on the scene, Colorado Springs police chief Richard Myers told a news conference.

Police did not identify the gunman. There were about 7,000 people in the building when shooting erupted, a pastor said.

In an earlier incident, 70 miles away, a man who entered a Christian missionary training centre in the Denver suburb of Arvada with a handgun killed two young missionaries and wounded two others shortly after midnight, police said.

The Arvada gunman, also dressed in dark clothing, fled on foot in the snow.

Police in the two cities said they were sharing information but declined to say whether they thought the attacks were related. There was no indication of motive in either case.

However a spokesman for the Arvada missionary group said the organization had an office on the Colorado Springs campus of the New Life church.

Myers said police had found several suspicious devices at the New Life church and were still searching campus buildings five hours after the shooting. He declined to elaborate.

The attacks -- at Christian religious buildings on a Sunday shortly before Christmas -- caused shock and dismay.

LEADING 'MEGACHURCH'

Colorado Springs is a focal point of evangelical activity in the United States. New Life is a leading "megachurch" with more than 10,000 members and the city is also the headquarters for the influential Christian conservative group Focus on the Family.

"When innocent people are killed in a religious facility or a place of worship, we must voice a collective sense of outrage," said Colorado Gov. Bill Ritter in a statement.

Democratic Sen. Ken Salazar of Colorado said, "It is incomprehensible that such atrocities could occur in places of faith and worship."

In the Arvada shooting, a young man came to the door of the Youth With a Mission dormitory asking for a place to stay, the group said in a statement.

When he was told he could not be accommodated there, he pulled out a handgun and opened fire. Two youth staffers were killed and two were wounded. They had been up late cleaning up after a Christmas party.

The mission is an international and interdenominational Christian organization that trains young people to work as missionaries.

In Colorado Springs, staff at the New Life church said they had tightened security after hearing about the Arvada shooting.

The roughly 7,000 people inside the building were swiftly evacuated to a downstairs basement after shooting started, pastor Brady Boyd told reporters.

"They came to church with their families to worship and what happened today was a real tragedy," Boyd said.

The New Life Church was founded 20 years ago by pastor Ted Haggard who resigned in disgrace a year ago after admitting to sexually immoral conduct following a friendship with a male prostitute.