A man accused in some of the freeway shootings that put Phoenix drivers on edge for weeks pleaded not guilty Thursday as his defense lawyers questioned the strength of the evidence against him.

Attorneys for Leslie Allen Merritt Jr., 21, who was arraigned on 15 felony counts, including aggravated assault and carrying out a drive-by shooting, said outside court that the investigation by state police does not place him at the shooting scenes.

"We're going to work diligently to make sure that we investigate this fully, and we believe in his innocence," said Ulises Ferragut, one of Merritt's two attorneys.

Ferragut and attorney Jason Lamm also cited investigators' evolving timeline of the shootings. They plan to do their own investigation, looking into another person possibly admitting responsibility for any of the 11 shootings, Lamm said. They didn't identify that person or provide details.

"It's very, very early in the game to get hard confirmation on that," Lamm said.

Department of Public Safety investigators used ballistics tests to tie Merritt to four of the 11 shootings that occurred on Phoenix-area freeways between Aug. 22 and Sept. 10.