Four months into his job, a police officer in Mississippi holds a gun to the head of an unarmed teenager and puts him in a chokehold. A rookie officer in Illinois gets into a car chase that kills a driver. And a new campus policeman in Indiana shoots an unarmed student to death.

Some are blaming these harrowing episodes on what an Associated Press survey found is a common practice across the country: At least 30 states let some newly hired local law enforcement officers hit the streets with a gun, a badge and little or no training.

These states allow a certain grace period — six months or a year in most cases, two years in Mississippi and Wisconsin — before rookies must be sent to a police academy. In many cases, these recruits are supposed to be supervised by a full-fledged officer, but that does not always happen.

The risks, some say, are high.

“You wouldn’t want a brain surgeon who isn’t properly trained. Someone shouldn’t be out there carrying a badge and a gun unless they are qualified to be out there,” said Jeremy Spratt, program manager of the Missouri Peace Officer Standards and Training Program.

No one seems to know how many untrained recruits are on the streets. But the practice appears to be most common among small-town police forces and sheriff’s departments.

Many police chiefs interviewed for this story said that for years, they have used less-than-fully-trained officers without problems, and they strongly defended the practice for reasons of money and manpower.

It allows departments to put new hires on the streets right away, without waiting for them to go through police academy training, which is usually a full-time, weeks-long, or months-long exercise during which the officer is not on duty but still on the payroll. In some places, there are waiting lists to get into the academy.

Also, some police forces see the grace period as a tryout, during which the department can decide whether the officer is going to work out before it invests thousands of dollars in police academy training. (In several states, if a recruit graduates from the academy, the police force is reimbursed by the state, but not if the officer fails to finish.)

“It lets the officer work for the department for an amount of time to make sure that’s what they want to do and make sure that’s the right person for the job,” said Batesville, Miss., Police Chief Gerald Legge. “We get some people that work a few weeks and say, ‘This isn’t what it was like on TV and this is not for me.'”

Chris Hollingsworth, 24, was hired two weeks ago by the Newton, Miss., police but is not scheduled to go to the academy until April. He said that he is working under the supervision other officers.

“I can see how [the grace period] would be a positive thing as far as letting people see if this is what they want to do for a living,” Mr. Hollingsworth said. “But I can also can see how it would be a negative thing because you’re a real big liability until you go through the training, and there’s not much you’re allowed to do.”

In 2003, Robert Duplain, a 24-year-old rookie police officer at Ball State University in Indiana, fatally shot a student — three rounds in the chest and one in the head. The officer is facing a wrongful death lawsuit.

Mr. Duplain had taken only a 40-hour “pre-basic course” consisting of mostly online classes and firearms training, said Rusty Goodpaster, director of Indiana’s police academy.