According to the Warren Report, there was "considerable witness testimony" that suggested shots were fired on President John F. Kennedy's motorcade from behind a picket fence atop the grassy knoll to the southwest of the Texas School Book Depository.
The report states that even after the Warren Commission completed their account of the assassination, two separate government panels had the X-rays and photographs taken during President Kennedy's autopsy examined by independent medical experts and they reached "the same basic conclusion: the President was struck by two bullets from behind."
The report indicates conspiracy theories exist about a second shooter on the grassy knoll because neither of the two separate examination groups published the X-rays or photographs and they didn't explain the basis of their conclusions in a public hearing -- so they didn't do anything to remove the doubt about a second shooter.
Acoustical evidence of police radios examined by the Warren Commission led some to believe there may have been four shots in Dealey Plaza, three from the book depository and one from the grassy knoll. But when comparing the acoustical data with the Zapruder film and other evidence, investigators could only conclude that both President Kennedy and Gov. John Connally were struck by one bullet and that the president was hit by a second bullet.
The Warren Commission concluded that the shots that killed President Kennedy and wounded Governor Connally "...were fired from the sixth floor window at the southeast corner of the Texas School Book Depository." (51) It based its conclusion on eyewitness testimony, physical evidence found on the sixth floor of the depository, medical evidence and the absence of "...credible evidence that the shots were fired from ...any other location."†
After 50 years, the grassy knoll remains a place of intrigue for those who believe the assassination of President Kennedy was organized by more than a lone gunman.
† Source: Archives.gov - Report of the Select Committee on Assassinations of the U.S. House of Representatives.