Police shot and killed a fellow officer when he fired his gun as they tried to take him into custody on suspicion of sexual misconduct with a teenage girl, authorities said.

The officer was on duty at a DUI checkpoint early Saturday when the shooting occurred, Santa Maria police Chief Danny Macagni said Saturday.

The four-year Santa Maria department veteran had just learned of the internal investigation of the alleged sexual relationship with a 17-year-old girl.

"We had no choice," Macagni said in video of a news conference posted by KCOY-TV. He said investigators had evidence "that demanded that we go out and take this officer off the street immediately."

Supervising officers were sent to make a felony arrest shortly after 1 a.m., but he struggled with them when they arrived, first putting up a physical fight, then firing his gun but hitting no one, Macagni said.

"He chose to resist, he drew his weapon, a fight ensued, he fired his weapon," the chief said.

Several officers came to help the police making the arrest, and one of them shot the suspected officer in the chest once, Macagni said.

He was declared dead after emergency surgery at Marian Medical Center, Macagni said in a statement.

Detectives had begun investigating the alleged relationship on Thursday night, and minutes before the shooting had confirmed that an "inappropriate" and "very explicit" relationship had been going on, Macagni said.

He said he could not give details because of the sensitivity of the investigation, but "there was some witness intimidation involved" and the arrest couldn't wait for a more proper time or place.

"The information that we had in hand demanded that we not let him leave that scene, get in a car, drive somewhere, it would put the public at risk," Macagni said at the news conference. "We just did not know what was going to happen, we did not expect him to react the way that he did."

Macagni said police had expressed condolences to the officer's family.

The officer who fired the fatal shot, an eight-year department veteran, has been placed on administrative leave, and the Santa Barbara County Sheriff's Department was investigating the shooting, Macagni said.

The name of the officer killed has not been released because some family members were still being notified, and the name of the officer who fired the shot was withheld while the incident was under investigation, police said.

Santa Maria is a city of some 100,000 people about 60 miles northwest of Santa Barbara and 160 miles northwest of downtown Los Angeles.