US presidential candidate Hillary Clinton warned about the dangerous combination of mental illness and guns during a campaign stop in Ames, Iowa on 26 July.

"We've got to do a better job," she said. "There are 128,000 people in this state alone who have serious mental health problems. And we see what happens when we have mental health problems and guns. We've seen it over and over and over again," she told a crowd of supporters at an organizing event at Iowa State University.

A 59-year-old man who had been committed to a hospital for psychiatric care was identified by authorities as the gunman who fatally shot two people in a rampage at a cinema in Louisiana on 23 July before turning the gun on himself as police closed in.

"We just saw it in a movie theatre in Lafayette, Louisiana. So this is not only a problem about people unable to take care of themselves or their families seeking help, but sometimes they are a danger to themselves and others. So we have to take it seriously," the Democratic candidate said.

During her campaign stop, she also reiterated her support for the nuclear deal the Obama administration has struck with Iran.

Clinton said she "didn't trust" Iran, but said the agreement is "better than the alternative".

"This is an important step in trying to put the lid on Iran's nuclear weapons program," she said. "Is it perfect? No. It was negotiated by human beings. But is it better than the alternative? Yes, it is."

Clinton is in Iowa for the third time this month, with seven events scheduled over the next three days. She's one of five presidential hopefuls stumping through the state this weekend.