A Puerto Rico policeman fatally shot two high-ranking officers and a policewoman on Monday following an argument at work that temporarily shut down the station in the U.S. territory's second-largest city, authorities said. The suspect was immediately placed under arrest.

The suspect, Guarionex Candelario, held a female lieutenant, a male commander and a policewoman hostage in an office before he killed them, police spokeswoman Mayra Ayala told The Associated Press.

She said authorities were about to start negotiations with the 50-year-old suspect when the victims were killed. She said police did not yet have information on a motive.

Officer Guarionex Candelario has been accused of fatally shooting three of his police colleagues in Ponce, Puerto Rico, on Monday. (Handout )

Gov. Alejandro Garcia Padilla told reporters that the suspect was injured during the shooting and remains in critical condition. Padilla declined to say what kind of injuries the officer received and how they were inflicted.

"This took us completely by surprise," Col. Hector Agosto said in a quiet voice at a press conference while still wearing a shirt stained with blood, explaining that he tried to help the victims. "We are a family. I am trying to stay strong."

"This is a very tragic situation that appalls us all especially because it occurred during the holidays," added police spokesman Axel Valencia.

The shooting occurred inside the lieutenant's office at the police headquarters in the southern coastal city of Ponce. Authorities evacuated the station to investigate threats of a possible explosive device in the suspect's car, but they did not say whether anything was found.

Police told reporters that the suspect previously had been stripped of his weapon but received a psychological evaluation nearly two years ago and his weapon was returned. Authorities declined to provide any other details, saying the investigation was barely starting. Candelario had been with the department for 19 years.

A police officer reportedly shot and killed three fellow officers Monday morning at the police headquarters in Ponce, Puerto Rico's second-largest city. (CBC)

"Ponce is in mourning," Mayor Maria Melendez said in a statement. "May God give the family members of the police officers involved the strength, fortitude and serenity to absorb this news."

Ayala said the suspect worked in the anti-drug division. Officials said the victims were Lt. Luz Soto Segarra and Cmdr. Frank Román Rodríguez, both 49, and policewoman Rosario Hernández de Hoyos, 42. Soto had been with the department 23 years, Roman 28 years and Hernandez 15 years.

Soto's husband also was a police lieutenant, said Jose Cruz Martinez, vice-president of the Puerto Rican Police Union.

Soto and Román Rodríguez each had been with Puerto Rico's police department for more than 20 years, said Cruz, adding that they had been his friends for 15 years.

"They were both excellent people," Cruz said in a phone interview. "They were always supportive of their coworkers."

Cruz said he and his wife would occasionally go on motorcycle road trips with Soto and her husband, who lived in the nearby town of Yauco. He said Roman was originally from the western town of Mayaguez.

"He rarely had any time off," Cruz said of Roman.

Valencia said Roman was named Commander of the Year for Ponce in 2014.

It is the largest shooting of its kind in Puerto Rico's history. The last shooting at a police station on the island occurred in September 2007, when a police sergeant in the southeast town of Yabucoa killed his supervisor after an argument about work scheduling.

Puerto Rico's police force is one of the largest in a U.S. jurisdiction. The troubled department is undergoing a 10-year federally mandated reform after U.S. prosecutors accused officers of illegal killings, corruption and civil rights violations.