JACKSONVILLE, Fla. -- Extra homicide detectives had to be called in Sunday as police responded to three fatal shootings -- one victim a 16-year-old boy riding a bike -- in less than seven hours.

A man was also stabbed multiple times in that same time period.

Just after 6 a.m., police responding to reports of gunfire found a man dead in the front yard of a home in the 1600 block of Academy Street.

Police said the victim, 39-year-old Patrick Stafford, was visiting relatives when he was shot. They had no suspects and no motive.

Four hours later, police were called to King Street, near the Hollybrook Apartments, after 16-year-old Monquell Wimberly was hit by gunfire while riding his bicycle.

Wimberly died at the scene.

Witnesses told detectives the shots may have been fired from a gray or tan four-door Mercury Marquis or Ford Crown Victoria.

Just before 3 p.m., 30-year-old Nathaniel Grant was shot in the chest at an abandoned home at East 27th and Buckman streets, just off Talleyrand Avenue.

Police said Grant made it about a block to the Williams Grocery, where the clerk called 911.

Grant was transported to Shands-Jacksonville Medical Center, where he died.

Police were also investigating after Daryll Alexander, 33, was stabbed several times about 10:30 a.m. Sunday near the intersection of 45th Street and Moncrief Road. He was hospitalized in critical condition.

Detectives said they had information about possible suspects in the stabbing.

"(It's) the black-on-black crime that we got to, just, get a hold of," said Charlene Austin, whose family owns the Williams Grocery. "It's just pathetic the way the young children, they have no regards for anybody's life."

Police told Channel 4's Diane Cho they have no reason to believe that any of the incidents were related.

Anyone with information that could help identify suspects in any of these cases is asked to call Crimestoppers at 866-845-TIPS.