A black bag containing ammunition found by a teen on the northwest side has been linked to accused gunman suspected gunman Jared Lee Loughner, officials say.

An 18-year-old man walking his dog near Loughner's residence found the bag in a wash Thursday morning, took it to a neighbor's house and called police, said Deputy Jason Ogan, a Pima County Sheriff's Department spokesman.

Deputies believe the bag could be the one Loughner took with him Saturday morning before the mass-shooting outside a Safeway.

Investigators are at the neighbor's house where the bag was taken, Ogan said.

Randy Loughner told investigators that he found his son near the front of the house Saturday morning removing a black bag from the family's vehicle. He had concerns about what his son was doing and about what he had in the bag, said sheriff's Bureau Chief Rick Kastigar.

"The father asked him something along the lines of 'What are you doing?' or 'What's in the bag?' " Kastigar said.

Loughner then mumbled something and quickly walked away, Kastigar said.

The father got in his vehicle and drove out to look for his son but he never found him.