Dawn Mcintosh was charged with killing her boyfriend, Shawn McWhorter, in an Astoria alleyway, police said. View Full Caption DNAinfo/Trevor Kapp

QUEENS — A woman fatally shot her boyfriend and then set him on fire behind an Astoria alleyway Saturday during an argument over his infidelity, police said.

Dawn Mcintosh, 46, has been charged with murder, weapons possession and tampering with evidence in the death of 43-year-old Shawn McWhorter, NYPD officials said.

Mcintosh was nabbed after police discovered a fingernail in McWhorter's clothing. When they interviewed her, they noticed she was missing a nail and later charged her, police sources said.

Mcintosh was arguing with McWhorter Saturday morning when she shot him in the left side of his body in an alley behind 27-18 14th St., near 27th Avenue, police said.

She then enlisted the help of her 21-year-old brother, Donte Watkins, who shared a nearby apartment with her, to try to move McWhorter's body, NYPD officials said.

They instead opted to set fire to his abdomen area at 6:57 a.m., police said.

Firefighters extinguished the blaze and McWhorter was pronounced dead at the scene, NYPD officials said.

"He was on the ground. He didn't have clothes. He didn't have a shirt. His pants were by his ankles. His hands were by his side. There was a lot of blood," said Cristobal Vermeo, 70, whose apartment overlooks the scene.

The city's medical examiner's office said he was killed by the gunshot and ruled his death a homicide, police said.

The brother and sister were arrested Sunday at 2 p.m. in the 114th Precinct stationhouse, an NYPD spokesman said.

Watkins faces charges of tampering with physical evidence, police said.

Mcintosh was previously arrested for assault in 1998, an NYPD spokesman said.