The weekend execution of an Iranian nuclear scientist mentioned in Hillary Clinton's private emails was "tragic," and there will likely be many more such incidents in upcoming weeks and months, retired Lt. Gen. Michael Flynn said Monday.

"It's just sad," Flynn, who had been on GOP nominee Donald Trump's short list for a running mate, told Fox News' "Fox & Friends" program. "I have a lot of problems, not only with this. Hillary Clinton's extreme carelessness in the use of private devices and a private server has placed U.S. national security at high risk."

Every "single thing" disclosed in Clinton's emails will have to be reviewed, he continued, and there will be an "economic and a human cost to that."

"There is an extraordinary cost to our nation's security and all sorts of other issues that are going to come up with this," said Flynn. "Not to mention what we don't know that she also was using in terms of the other 30,000 emails that may have something to do with other security issues that surround our country."

Flynn also spoke out about former CIA director Michael Morell's endorsement of Democratic nominee Clinton, and said that like Morell, he has worked for several administrations, but he cannot take "people who are standing up for someone who is a patent liar."

"Look at what she has said, just even recently in this whole issue, with trying to walk away from what she did with our nation's security," said Flynn. "So for Mike Morell to stand here and apologize in a way and to talk about Donald Trump, I just think that it's wrong.

"He can do what he wants, but he basically shown his true colors."