Four persons, including a gunman, were killed and six injured in two shootings at a church and a missionary training centre within 12 hours of each other in the US' Colorado state, just three days after a firing spree by a youth left nine people dead in a Nebraska shopping mall.
A gunman shot dead two staff members of missionary training center Youth for Mission staff and injured two others at 12.30 am in Arvada after being denied permission to spend the night there, before escaping on foot.
Just 12 hours later, another man was shot dead and four others injured by an attacker near New Life church before he was killed by a security staff in Colorado Springs, about 112 kilometers away from the first shooting site.
The assailant, who matched the description of the Arvada attacker, opened fire as soon as about 7,000 worshippers completed their late morning service.
Arvada police chief Don Wick said they had "reasons to believe" that the two shootings may be related but refused to confirm if the same gunman, said to be a white male in his 20s, was involved in the incidents.
Police backed by a canine unit followed the footsteps of the gunman in Arvada for about three blocks and then lost them. It was not clear whether he had escaped in a vehicle.
A young woman and man were among those killed while another shot in the neck was in critical condition. The fourth victim was hit in the leg, police said.
In Colorado Springs, Police Chief Richard Myers praised the courage of the security man who shot dead the gunman, saying he probably saved many lives.
The Colorado church, which is said have a membership of 10,000 despite a controversial history as its founder was oustedlat year after admitting to "sexual immorality", was evacuated after police found several suspicious devices and searched the huge campus room by room.
An elite police team, along with an explosive unit was clearing the multiple buildings at the New Life Church, Myers added.
But it was not clear whether the shooting was random or if there was motive, police said.
The two shooting come within four days of a youth killing eight people before turning the gun on himself in a crowded shopping mall in Omaha.
While recent shooting rampages in the country have reignited the debate over its lax gun control laws, Sunday's shootings at religious places have evoked outrage as also fear among the congregations of possible copycat incidents.
Los Angeles police arrested a 21-year-old university student on Saturday night in connection with an online threat to shoot people on campus.
Carlos Huerta was arrested near his apartment on Loyola Marymount University campus for allegedly posting a message on a chatboard that he would shoot and kill as many people as possible on campus before being killed himself by police.
Rev Brady Boyd, senior pastor of New Life Church, said the church had a security plan in place. "I'm proud of the way our team responded today," he said.
"Many, many lives were saved because of the quick action of some committed volunteers at our church."
Youth with a Mission, which is commonly referred to as YWAM, is a group that trains people to be missionaries from all over the world.
"The young man I don't think (the victims) knew him but he must be going through a lot personally in his own life to do something like this," a police officer said.