(Reuters) - A 28-year-old convicted killer who had abandoned his appeals process was executed in Delaware on Friday after a failed effort waged by federal public defenders to spare him, authorities said.

Shannon Johnson, 28, died by lethal execution at 2:55 a.m., said Brian Selander, a spokesman for Governor Jack Markell. He is the fifteenth U.S. prisoner executed in 2012.

By law, executions in Delaware must take place between 12:01 a.m. and 3 a.m. Inmates have the choice of hanging or lethal injection.

Johnson's execution came after the U.S. Third Circuit Court of Appeals twice overturned stays granted by a district judge.

Johnson was convicted of the 2006 murder of Cameron Hamlin, 26, who had been dating Johnson's ex-girlfriend. Johnson had hoped for a reconciliation, officials said.

Sentenced to death in 2008, Johnson had decided to forgo further appeals, Selander said.

The federal public defender's office, however, appealed on his behalf, once on Thursday and again early on Friday, he said.

There are 15 inmates on Death Row in Delaware, which had last executed an inmate in July 2011.

Selander said demonstrators both for and against the death penalty were gathered outside the state penitentiary at Smyrna during the execution.

Johnson's execution is the second this week in the country and the fifteenth so far this year, according to the Death Penalty Information Center, a research center focused on capital punishment.

On Wednesday, the state of Ohio executed Mark Wiles, 49, who fatally stabbed the 15-year-old son of his former employers.

(Reporting by Dave Warner; Editing By Ellen Wulfhorst and Vicki Allen)