The crime rate among illegal immigrants in Arizona is twice that of other residents, Attorney General Jeff Sessions said Friday, citing a new report based on conviction data.

The report, from the Crime Prevention Research Center, used a previously untapped set of data from Arizona that detailed criminal convictions and found that illegal immigrants between 15 and 35 are less than 3 percent of the state’s population, but nearly 8 percent of its prison population.

And the crimes they were convicted of were, on the whole, more serious, said John R. Lott Jr., the report’s author and president of the research center.

His findings also challenge the general narrative that immigrants commit fewer crimes. Those past studies usually don’t look at legal versus illegal populations, Mr. Lott said.

Mr. Lott said the Arizona data is able to peek behind that curtain, and the differences between the populations were stark.

“There appears to be a huge difference between the two groups,” Mr. Lott said. “The type of person who goes through the process to legally immigrate in the United States appears to be very law-abiding versus even the U.S.-born population. The reverse is true for undocumented immigrants — they are committing crimes, and more serious crimes.”

Among nearly 4,000 first- and second-degree murder convictions, undocumented immigrants accounted for nearly 13 percent — significantly higher than their percentage of the population. Legal immigrants, by contrast, were less than 1 percent of convicts. Native-born made up the rest.

Undocumented immigrants also accounted for five times the rate of convictions for money laundering and kidnapping, and were three times more likely to be convicted of drive-by shootings.

The data covered from 1985 to 2017. For his data purposes, Mr. Lott defined undocumented immigrants as those who weren’t U.S. citizens or green card holders, signaling permanent residency.

He said the crime rates of the undocumented who were ages 18 to 35 was particularly important, given the ongoing debate over legalizing illegal immigrant “Dreamers.” He said the Arizona data showed that population had crime rates 250 percent higher than their share of the population would have predicted.

Mr. Lott said told The Times that the data belonged to Arizona and his agreement was that he wouldn’t give out the raw data without their approval.

Mr. Sessions’ citation of the data Friday inserted the information directly into the ongoing immigration debate.

“They’re more likely to be convicted of sexual assault, robbery, and driving under the influence. They’re more than twice as likely to be convicted of murder,” Mr. Sessions said in remarks prepared for a speech in Norfolk, Virginia.

He continued: “Tens of thousands of crimes have been committed in this country that would never have happened if our immigration laws were enforced and respected like they ought to be.”