Could Ariel Castro be tried for murder? Case would be unprecedented.

Aggravated murder charges likely will be sought against Ariel Castro, a prosecutor said. Experts say it is unprecedented to sentence someone to death for killing a fetus in a case in which the mother survives.

When Ariel Castro was arrested this week, he was charged with the kidnapping and rape of three women he allegedly held captive for a decade in his Cleveland house.

Then came the announcement Thursday by Cuyahoga County Prosecutor Timothy McGinty that his office will likely press additional charges of aggravated murder related to allegations that Mr. Castro beat Michele Knight so severely that she miscarried at least five times.

In Ohio, the charges of aggravated murder would make Castro eligible for the death penalty, which is reserved for the “most depraved criminals who commit aggravated murder during the course of a kidnapping,” Mr. McGinty told reporters Thursday.

But legal experts say it is unprecedented to send someone to Death Row for killing an unborn child in a case in which the mother survives. For example, Scott Peterson was sentenced to death in 2005 after being found guilty of both the second-degree murder of his unborn son and the first-degree murder of Laci Peterson, his wife.

“Nobody has ever been prosecuted [in] a full-fledged death penalty case based on pregnancy termination all the way through,” says Douglas Berman, a professor of law at Ohio State University in Columbus who is an expert on sentencing law and policy. “It’s a very hard case to prove and establish.”

The challenge in the Ohio case, experts say, is proving Castro intended to kill the fetuses, or even knew Ms. Knight was pregnant at the time he allegedly beat her.

“It’s going to be very difficult. Without a confession, or circumstantial evidence, it’s going to be very tough,” says Deborah Denno, a law professor at Fordham School of Law in New York City.

According to the National Conference of State Legislatures, at least 38 states have fetal homicide laws that impose criminal penalties for crimes against pregnant women, and 23 states, including Ohio, expand the law to cover the earliest stage of pregnancy, such as conception or fertilization.

Lisa Smith, a professor at Brooklyn Law School who specializes in domestic violence cases, says that “there is usually more proof” in successfully proving these cases. It is not known, at this point, whether there is any physical evidence that Castro beat Knight or whether there were any miscarriages.

“This is different because the mother is alive and the fetus, or several fetuses, are dead,” Professor Smith says. “The prosecution is more likely successful with the death of the mom along with the death of the unborn child.”

If the aggravated murder charges are brought, prosecutors would need to rely on a thorough medical examination of Ms. Knight to search for bruising, plus testimony from Gina DeJesus and Amanda Berry, the two other women held captive in the house.

Both potential sources of information are fraught with challenges. All three women are considered to be in a highly fragile state following 10 years of physical torture and psychological abuse, plus police say they were often separated from one another in rooms, which suggests that they were not fully aware of the circumstances of the other.

Another question, according to Smith, is if Knight were beaten so severely as to induce a miscarriage, would she not have required medical attention? The women hardly left the house.

With those roadblocks ahead of them, says Ohio State’s Professor Berman, prosecutors may be using the death penalty as a tactic to coerce Castro to plead guilty to the multiple kidnapping and rape charges he already faces.

“This kind of huffing and puffing does a nice job in convincing the public the prosecutors are taking this very seriously, and it makes it very easy for them to say to Castro it’s time to plead guilty for something that will put you in prison for the rest of your life,” he says.

If the prosecutors pursue the murder charge against Castro, they will need to take into account the toll it will take on the other former captives, because the case will rely mainly on their testimony in court.

“If I was the prosecutor right now, my first questions would be, ‘if I pursue the death penalty, what will that put the victims through? I would sit down the victims and ask, ‘What would you like me to do?’ ” Berman says.

Castro is charged so far with four counts of kidnapping and three counts of rape.