An unidentified man armed with a gun has been shot dead at the offices of Colorado Governor Bill Ritter.
The gunman said he was going to take over state government
The man, who claimed he was "the emperor", said he was going to take over the state government.
The incident occurred at about 1415 (2015 GMT) in the foyer. Mr Ritter was unhurt, his spokesman told the BBC.
The man was ordered repeatedly to put down his firearm before he was shot by state troopers, who provide security at the Denver state capitol.
Four or five shots were heard, but authorities would not say how many times the security officer fired. Police said the unidentified man did not use his weapon.
'A bit rattled'
Gov Ritter was in the capitol at the time but his whereabouts were not disclosed.
"The governor is fine. Everybody is a bit rattled," said Gov Ritter's spokesman Evan Dreyer.
Edward Casso, a state representative who was at the scene shortly after the shooting, said the gunman was in his 30s or 40s, wearing a white shirt and dark trousers.
The state capitol was reported to be sealed off while police carried out a room-by-room search of the building.