The mayor has also expressed outrage over the training shooting and has vowed that the incident will be investigated fully.

Since coming to Baltimore in September, Batts has made several leadership changes, including in training operations, as he seeks to put his stamp on the department.

Batts quickly stopped training at the city police academy after the incident, though some non-shooting exercises are expected to resume this week.

Police have not identified the injured trainee, a campus officer in his 40s who was in serious condition Saturday night. Eighteen-year veteran William Scott Kern, 46, has been named as the instructor who fired the bullet.

City police have released few other details, but sources familiar with the investigation say state police are looking into the possibility that Kern accidentally reached for his service weapon instead of a paint-cartridge pistol and pointed it playfully at recruits before the shooting.

The Baltimore department prohibits the use of service weapons and real bullets in training environments, and it remains unclear why Kern had a real gun.

State police will forward their findings to Baltimore County's state's attorney, who will determine whether to file criminal charges.

Scott, the law enforcement consultant, said the involvement of a police instructor raises more concern, because instructors are charged with ensuring safety regulations are followed.

Training experts say no gun — even a practice weapon — should be fooled with. Many departments do not allow training pistols to be pointed at trainees outside of controlled exercises.

"There can be absolutely no horseplay in a firearms-training-related site," Scott said.

Guglielmi acknowledged that many things went wrong Tuesday and said the department is "outraged at what happened."

"It shows at minimum poor leadership decision making by the instructors of this group," he said.

Departments that have experienced accidental shootings during training have focused on ways to prevent actual guns from being mixed up with practice weapons.

Such mix-ups have cost several officers their lives.

St. Joseph, Miss., police officer Dan De Kraai was killed in 2010 after he told a fellow officer that he wanted to know what it felt like to be shot with training ammunition. The other officer obliged, forgetting that he had picked up his service weapon during a break.

Georgia probation officer Tiffany Danielle Bishop, 24, was killed a year later during a prison training class when a firearms instructor pointed his gun at Bishop and pulled the trigger.

According to news reports, the instructor had returned from a lunch break without going through the proper safety checks to make sure he wasn't carrying a loaded gun. The instructor pleaded guilty to involuntary manslaughter and was sentenced to 90 days in jail with 10 years' probation. His law enforcement certifications were revoked.

While a friendly-fire incident can traumatize agencies, accidents can also help prevent future mistakes, as in the case of Cpl. Joseph Cushman.

Police in Arlington, Texas, were practicing school shooting scenarios at a junior high on June 7, 2001. All of the officers wore helmets and vests, and were supposed to be using training ammunition.

During a demonstration to SWAT trainees, an instructor's 9 mm Gluck discharged, firing a live round that struck Cushman in the head, killing him. An investigation showed that instructors had been allowed to bring loaded weapons to the exercise and that the instructor thought he had fired a training weapon.