GOP presidential nominee Donald Trump issued a challenge to Russia at a Miami press conference on Wednesday.

“Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing. I think you’ll probably be rewarded mightily by our press.”

It is not clear what set of emails Trump is referring to.

There are two email controversies swirling around the Hillary Clinton campaign. There are the missing emails from her private server while she was serving as secretary of state. But FBI Director James Comey said earlier this month that the investigation into those emails was over; he found no evidence to indict Clinton.

There’s also the hack into the Democratic National Committee’s emails, which forced chair Rep. Debbie Wasserman Schultz to quit. US intelligence officials and others have said there are strong indiciations the hack was perpetrated by Russian intelligence agencies.

Regardless of which set of emails Trump is referring to, his comments have left experts alarmed. Former US ambassador to Russia Michael McFaul says he's shocked.

"He's basically encouraging the Russian intelligence services to steal information from a former American official," he says. "I can't remember any American ever suggesting that we should help the Russians steal our information, let alone one running for the president of the United States."

McFaul adds that these kinds of comments make him very nervous about American leadership in the world. But ultimately, he says, it's up to American voters to decide whether they want Trump to be defending US interest.

"[The Russian hacking] has happened a while ago, so I'm just assuming and hoping that [US officials have] been investigating ever since we knew about this," McFaul says.

As a former ambassador to Russia, he believes the US officials are looking at the DNC hacking issue very carefully.

"Let's not be naive," he says, "this is not the first attack against US government officials or entities or American organizations."

That's why, he says, it's very likely that the entire embassy is wondering about other attacks and ways to be more vigilant.