A man returned to his Newark home Monday night to find his longtime girlfriend and the mechanic she was having sex with dead after they were overcome by carbon monoxide in the couple's garage, a police department source told NJ Advance Media.

The 41-year-old woman was apparently paying for work on her car by having sex with the 56-year-old mechanic, NBC New York reported. Newark police spokeswoman Catherine Adams said, however, that the department has no information about how the accident occurred.

Adams said police responded to the apartment complex at 18th and New Fairview avenues on a call of "sick or injured persons" just before 8:45 p.m.

Officers discovered the man and woman in a running vehicle inside the closed garage, and that both were pronounced dead at the scene, she said.

"This incident is under investigation, but appears to be accidental," Adams said Monday night.

Newark Fire Division evacuated the building to protect other residents, and one person was taken to the hospital for evaluation, she said.

(Editor's Note: A previous version of this story misattributed details about the sexual encounter to a police source.)

Chris Sheldon may be reached at csheldon@njadvancemedia.com. Follow him on Twitter @chrisrsheldon Find NJ.com on Facebook.