The shooting Thursday in Chattanooga, Tennessee, that left four U.S. Marines dead could be attributed to the presence of military gun-free zones, Republican presidential candidate Donald Trump suggested Friday morning during an interview. Trump, who was leading polls for the GOP presidential nomination, said that if the soldiers had been armed, the shooting would not have led to the four fatalities.

"We have to start doing something with these gun-free zones," Trump said on MSNBC's "Morning Joe." "They were sitting there and they were just targets for this madman."

Mohammod Youssuf Abdulazeez was identified Thursday as the gunman who attacked a military recruitment center and a naval reserve center seven miles away, killing four and injuring seven people before being fatally shot by law enforcement. 

Trump also suggested that a lack of an aggressive police force also contributed to the shooting. "We're going to have to get much tougher on law enforcement," Trump said, adding that officers were now "scared" to approach people for fear of losing their jobs. His statements came one day after U.S. President Barack Obama visited a federal prison as part of a broader push for criminal justice reform and diminished police brutality.

"As commander-in-chief, I would mandate that soldiers remain armed and on alert at our military bases," Trump told July 7. He said he was a lifelong member of the National Rifle Association.

Although military police are armed, Trump said soldiers were forbidden from carrying guns on bases because of a law passed under former President Bill Clinton.

Trump also criticized Obama on Thursday on "The O'Reilly Factor" for refusing to identify Islam as a cause of terrorism, and suggested that terror attacks would grow because of a lack of respect for America's law enforcement. "If we're going to fight elements that are causing tremendous problems," Trump said, "at least we have to start maybe being not so politically correct."