The incident occurred at the victim's home on the 3100 block of Bermuda Drive around 7:30 p.m.
Police responded to the home following reports of a man down in the front yard of his home around 5:45 p.m. When officers arrived, the man was very confrontational and declined their offer of help. Neighbors said they also tried to convince the man to go to a hospital but he refused and went inside his home.
"Our neighbors were out there talking to him, the ambulance came, the cops came," said Laurie Raphoon. "They were trying to take him to the hospital to see if he was OK, what was wrong with him."
Nearly two hours after authorities left the scene, neighbors heard a loud explosion, and the man was found dead.
The FBI and Costa Mesa police were summoned to the location.
Personnel from the Orange County Sheriff's bomb squad and Orange County Fire Authority Hazardous Materials unit also rushed to the scene.
Police suspect he was killed by some sort of homemade explosive, but investigators had not determined whether the blast was suicide or an accident.
At least two other explosive devices, identified as pipe bombs, were later found at the home, police Sgt. Jerry Hildeman said.
"There was one upon his person, one nearby, so we believe he was possibly wearing two and one contained within the house," Hildeman said.
The bomb squad detonated the third improvised explosive device. Police evacuated residents from 16 homes as a precaution as officers continued to search for other explosives on the property.
Kat Genovesio heard the bomb blast Sunday that killed her neighbor.
"It was louder than fireworks," Genovesio said. "It was a big boom."
Authorities said the man was not fond of government agencies or law enforcement. No other information was released but neighbors say officers were often called to the home.
Neighbors said the man kept to himself and seemed strange but they had no idea there were explosives inside his home.
"It's really creepy having them pull out all of these explosives when you never really knew that that was there," Genovesio said.
In the meantime, police are escorting residents to their homes to get medication, clothing, or check on their pets while the evacuation order remains in effect.
The victim's name has not been released.