Detectives with the Chicago Police Department are working "around the clock" to find the shooter who gunned down an off-duty police officer, an official tweeted Sunday.
Officer John Rivera, 23, was shot and killed early Saturday, a few hours after he finished his shift and handled paperwork for a homicide earlier in the night, police said. He was with another off-duty officer and two friends when a gunman opened fire on the car they were in, officials said Saturday.
(MORE: Off-duty Chicago police officer John Rivera ambushed, killed in 'unprovoked' shooting: Officials)One of Rivera's friends was also shot and wounded, police said. On Saturday, police said that the victim, an unidentified man, was in critical condition and undergoing surgery.
The shooter and at least one other suspect remained at large Sunday.
(MORE: 19-year-old woman arrested for allegedly shooting a Chicago police officer during raid)But a spokesman for the police department, Anthony Guglielmi, tweeted Sunday that investigators were working non-stop to try to find the killer.
"Investigators have been working around the clock to follow evidence & review dozens of cameras feeds," Guglielmi wrote.
Eddie Johnson, the department's superintendent, said they would be poring over the network of cameras in the downtown area where the shooting occurred.
"Mark my words," he said Saturday during a news conference, "we will find them."
Rivera and the other officer, who had just finished a "tour of duty," left a nightclub with their two friends at about 3:30 a.m. Saturday and returned to their vehicle, officials said. Two men approached their vehicle, and one opened fire, killing Rivera and wounding one of the friends.
The other off-duty officer and a woman with the group were not injured, police said.
The Chicago Police Department tweeted photos showing a procession of police vehicles transferring the officer's body to the Cook County medical examiner's office.
A person of interested was interviewed Saturday. Guglielmi said detectives had identified other people.
"We continue to identify persons of interest based on the investigation and we are speaking to those individuals," he tweeted.
ABC News' Alexandra Faul and Olivia Rubin contributed to this report.