By Jason Morris and Darran Simon, CNN

(CNN) -- Multiple Houston police officers were shot Monday while serving a search warrant, Police Chief Art Acevedo said.

About a dozen narcotics officers, along with several patrol officers, had responded to a residence where police suspected drugs were being sold, Acevedo told reporters late Monday night. The narcotics officers announced themselves as they breached the front door shortly before 5 p.m. Then, the gunfire started, Acevedo said.

"Immediately upon breaching the door, the officers came under fire from one or two suspects inside the house," Acevedo said.

One suspect retreated to the back of the room, the police chief said, then re-emerged and again returned fire.

Four undercover narcotics officers were struck by gunfire, police said. A fifth officer suffered a knee injury, according to Acevedo.

Police returned fire, killing two suspects at the scene, the police chief said. He did not release the suspects' names.

Acevedo said authorities first believed five officers were shot. Joe Gamaldi, president of the Houston Police Officers' Union, also initially said five officers were shot.

Three of the four officers who had been shot were still hospitalized, police said Monday night. Another officer who suffered a knee injury was also being treated at the hospital, police said.

"This has been a tough day for our city," Mayor Sylvester Turner said, encouraging residents to pray for the officers.

"Pray for their families, pray for their spouses, their children, their parents, all of their loved ones," the mayor said.

"This evening's horrific attack on police officers is a solemn reminder of the service and sacrifice our brave men and women in law enforcement make every day to keep us safe," Texas Gov. Greg Abbott said.