DENVER - Police in Colorado were looking into possible links between two church-related shootings in the U.S. Christian heartland over the weekend that left four victims and one of the gunmen dead.

The initial death toll was four in Sunday's shootings, including the gunman, but one of the people wounded at the New Life evangelical church in Colorado Springs died late in the evening, media outlets quoted hospital officials as saying.

A man dressed in black, wearing combat boots and holding an assault rifle and at least one handgun, opened fire in the parking lot of the vast New Life church after Sunday services, killing one person on the spot and wounding others.

A security guard then shot and killed the gunman, 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 the shooting erupted, a pastor said.

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

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

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

They came just four days after a 19-year-old man killed eight people and then himself with an assault rifle at a busy shopping mall in Omaha, Nebraska.

Police in the two Colorado cities were sharing information but there was no indication of the motive in either case.

While Myers said there was no information that the attacks were related, Arvada Police Chief Don Wick said there could be a connection.

"Yes, there is a reason to believe that, although this is active and I can't release that," Wick told reporters. "Until we have evidence which conclusively ties these two events together, Arvada police will continue to follow all leads."

A spokesman for the Arvada missionary group said it had an office on the Colorado Springs campus of the New Life church.

CNN reported Arvada police helped execute a search warrant with Colorado Springs police at an address in another suburb of Denver.

Myers said police had found several suspicious devices at the New Life church, but gave no details.

"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 of 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," Colorado Gov. Bill Ritter said in a statement.

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, he opened fire, killing two youth staffers and wounding two who had been cleaning up after a Christmas party.

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