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(CNN) —  

Houston police had previously bought black tar heroin at the home, Chief Art Acevedo said Tuesday. When a narcotics unit raided the house, officers swiftly came under fire, and a pit bull charged the first officer through the door, he said.

About a dozen narcotics officers and several patrol officers responded to a residence where police suspected drugs were being sold, Acevedo told reporters 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,” the police chief said.

One suspect retreated to the back of a room, he said, then re-emerged and 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 chief said. He did not release the suspects’ names.

Acevedo said authorities first believed five officers were shot.

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

Two of the hospitalized officers are in critical condition, while the other two are in good condition, police said. An officer who was discharged suffered a gunshot wound to the shoulder.

“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,” he said.

Texas Gov. Greg Abbott 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.”

CNN’s Samira Said, Nicole Williams, Janet DiGiacomo, Dave Alsup and Andrea Diaz contributed to this report.