Hours later, Boulder police issued a second tweet saying officers were responding to a report of an "armed, unsafe individual" at a nearby location, and that investigators were seeking to determine "if this is related to (the) King Soopers shooting".
Several people have been killed in a supermarket shooting in the USA state of Colorado after a gunman opened fire.
Authorities said efforts to identify the other victims and notify their families were ongoing.
According to American media, the gunman fired from an AR-15 machine gun, a very common weapon in the United States and used by a large amount of the perpetrators of mass killing operations in the United States.
Herold said police engaged in a shootout with the suspect inside the supermarket and that is when Officer Eric Talley was killed.
The public prosecutor promised to bring "justice" to all the victims, describing them as "people who were living their lives, who were shopping, before the gunman tragically destroyed their lives".
Boulder Police Chief Maris Herold said Talley was one of the first officers to respond to a 911 call from the grocery store and encountered the suspect, who was armed with a high-powered rifle.
Dozens of police and emergency vehicles, their lights flashing, escorted an ambulance carrying the slain officer from the shooting scene after nightfall.
The attack in Boulder, about 25 miles (40 kilometers) northwest of Denver and home to the University of Colorado, stunned a state that has seen several mass shootings, including the 1999 Columbine High School massacre and the 2012 Aurora movie theater shooting.
Monday's attack comes a week after the killing of eight people at three Atlanta-area massage businesses. "There are commonsense gun legislation reform proposals that have been debated in Congress for far too long", Neguse said.
"Our hearts go out to Officer Eric Talley, his family and the families of the other victims from this tragic day".
'We will continue to tweet updates as we have them.
Moonshadow told The Denver Post they ducked and "just ran".
Nicholas said police were speeding into the lot and pulled up next to the body.
What sounds like two gunshots ring out at the beginning of the video.
Investigators had just started sorting through the crime scene and conducting witness interviews, Dougherty said.
The FBI and agents of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting police and sheriff's deputies in the investigation, authorities said.
"I also want to stress how incredibly sorry I am for all the victims who were killed in Kings Soopers", Mr Dougherty said.