Donald Trump. AP Photo/Seth Perlman Real-estate mogul Donald Trump suggested Monday that former US Secretary of State Hillary Clinton would have serious legal problems if he gets to appoint the next US attorney general.

Speaking at a presidential campaign rally in Springfield, Illinois, Trump assailed Clinton for her controversial email practices at the State Department. Trump asserted that Clinton committed a "crime."

"You better remember: There's a six-year statute of limitations on that crime. So Hillary's running for a lot of reasons. One of them is because she wants to stay out of jail. Because I am sure — and first of all, everybody gets a fair shake with me — but I am sure whoever the attorney general is, you've got a lot of years left on that crime. That's a crime," he said.

"If I win, we're going to look into that crime very, very seriously, folks," he added. "She's watching right now and she's saying to herself, 'Man, I better win.' So we have to make our country great again. We have to do it. We're going to make it so good."

Clinton exclusively used a private email server and account for her government work in the Obama administration, an issue that has dogged her campaign since the spring.

The FBI has said that it is looking into whether any sensitive information was mishandled in connection to the account, but Clinton has not been accused of any wrongdoing and is not a target in the investigation.

After weeks of negative headlines, the Democratic front-runner apologized in September for her email arrangement, but insists that she didn't violate protocol or pass along material marked classified. Some of her messages were retroactively marked classified before being released to the public.

But Trump, no stranger to aggressive campaign-trail claims against his rivals, has declared that Clinton shouldn't even be "allowed" to run for president because of her former email system.

"If we had honest government, Hillary wouldn't be allowed to run," he said Monday. "She wouldn't be allowed to run. You know that. People are in jail right now for doing 5% of what she did. And the Democrats are not going to prosecute her. And it's frankly a disgrace and you know it."