Former Arizona US attorney speaks on Trump investigation

"There is no way Bob Mueller could do his job without a grand jury," Paul Charlton said.

Former U.S. attorney for Arizona, Paul Charlton says the grand jury allows special counsel Bob Mueller to compel witnesses to testify, and gives him broad access to records and documents.

This includes President Trump's taxes, which have never been released.

"So his taxes could be in play?" I asked.

"Almost certainly would be, I would think," Charlton said.

But Charlton cautions against reading too much into the grand jury.

"I know Bob Mueller, I think he's going to be careful, cautious, and there isn't going to be a charge brought here unless he genuinely believes that he can win at trial," he said.

Charlton says Mueller is the perfect person to oversee the investigation into the Trump campaign and any possible coordination with Russia.

"The saving grace here in terms of the system working correctly is Bob Mueller," Charlton said. "This is a squared away, seasoned prosecutor who knows that it isn't sufficient just to find probably cause."

Charlton says Mueller, a former Marine, will not be influenced by Washington politics.

"He doesn't care a wit about what it is he's hearing outside of that office," he said. "He's doing what he thinks is right."

"You trust what he finds will be the truth?" I asked.

"I trust Bob Mueller to do what he believes is right, yes," Charlton said.