By Gabriella Borter and Shivani Tanna

(Reuters) -Two people were killed and 28 others were injured, including four children, in a mass shooting in Baltimore, Maryland, on Sunday, according to police and hospital officials.

An 18-year-old woman and 20-year-old man were killed and three victims were still in critical condition Sunday morning, according to police in Baltimore, a city about 40 miles (64 km) north of Washington, D.C.

At least four victims were treated in the University of Maryland Medical Center’s Pediatric Emergency Department, the hospital said, without providing details on their ages.

The suspect or suspects were still at large.

“This investigation is ongoing, and we will not rest until the people responsible are held accountable,” Mayor Brandon Scott and the police department said in a joint statement.

The tragedy rattled the city at the start of the Fourth of July holiday weekend, when Americans typically gather for parades, barbecues and fireworks. The shooting occurred shortly after midnight at the “Brooklyn Day” block party in the Brooklyn Homes neighborhood of Baltimore, which hundreds of people attended, local media reported.

A witness told TV station Fox 45 that they had heard 20 to 30 shots fired.

A local reporter posted an image to Twitter showing police tape, cups and other litter strewn across a yard that appeared to be the crime scene.

Nine people were transported from the scene to local hospitals, while 20 others affected walked into hospitals in the area, police said.