Was it child abuse or an undiagnosed medical condition that killed a 3-month-old boy left with a home day-care provider in 2007?

Prosecutors say the case against Jennifer Hancock, 38, of Verona, is complex and circumstantial, but evidence will show she was stressed by Lincoln Wilber’s constant crying and caused the head injury that took his life.

“She killed him,” Assistant District Attorney Mary Ellen Karst told a jury of nine women and five men chosen Monday morning to decide the first-degree reckless homicide case against Hancock.

But it isn’t that simple, said Hancock’s attorney, John Hyland. He said Lincoln died from a condition inside his skull that worsened, undetected, until reaching a breaking point on Sept. 7, 2007, while at Hancock’s home.

While the case is circumstantial, Karst told the jury, it makes sense. Lincoln had eaten his breakfast and was happy, smiling and playing with his mother before he was dropped off at Hancock’s house that morning.

He was the baby his parents, Erin and William “Benjy” Wilber, hoped for when they married and settled in Belleville in 2006.

“By all accounts he was a healthy and happy little boy,” Karst said.

But only five hours after being left in Hancock’s care for the day, he was near death, Karst said. He was not breathing and had no pulse when police and paramedics responded to Hancock’s 911 call.

Lincoln died on Sept. 11, 2007, after life support was withdrawn. Karst said that although doctors found no external injuries, they found some old subdural hematomas and one new one — essentially bleeding inside the skull below the layer called the dura. Doctors also found Lincoln suffered a twisting or torquing fracture to his left leg, Karst said.

Medical experts will testify that Lincoln was the victim of abuse, Karst said, and his symptoms would have appeared only shortly after the abuse occurred, while he was at Hancock’s home.

But Hyland said jurors should believe the account Hancock gave to a 911 dispatcher and police that day — that she laid him down for a nap and returned a few minutes later to find him unresponsive.

Hancock also reported that another of the children in her care, a 3-year-old, had fallen on Lincoln as he lay on the floor for a diaper change.

That impact may or may not have triggered a condition already occurring in Lincoln’s skull that caused the hematomas, Hyland said. His medical expert, Dr. Ronald Uscinski, a Washington, D.C.-area neurosurgeon, will testify that Lincoln died from a re-bleed of a chronic subdural hematoma, a condition that can occur spontaneously.

Hancock’s trial is scheduled to last two weeks.