Concerns are growing over the possibility of a rigged presidential election. Experts believe a cyberattack this year could be a reality, especially following last month's hack of Democratic National Committee emails.

The ranking member of the Senate Homeland Security Committee sent a letter Monday to the Department of Homeland Security, saying in part: "Election security is critical, and a cyberattack by foreign actors on our elections systems could compromise the integrity of our voting process."

Roughly 70 percent of states in the U.S. use some form of electronic voting. Hackers told CBS News that problems with electronic voting machines have been around for years. The machines and the software are old and antiquated. But now with millions heading to the polls in three months, security experts are sounding the alarm, reports CBS News correspondent Mireya Villarreal.

For weeks, Republican presidential nominee Donald Trump has told his supporters the outcome of the 2016 election could be out of his control.

"I'm afraid the election is going to be rigged, I have got to be honest," Trump said to Ohio voters last week.

But for the hackers at Symantec Security Response, Election Day results could be manipulated by an affordable device you can find online.

"I can insert it, and then it resets the card, and now I'm able to vote again," said Brian Varner, a principle researcher at Symantec, demonstrating the device.

Brian Varner and Mireya Villarreal CBS News

The voter doesn't even need to leave the booth to hack the machine.

"For $15 and in-depth knowledge of the card, you could hack the vote," Varner said.

Symantec Security Response director Kevin Haley said elections can also be hacked by breaking into the machines after the votes are collected.

"The results go from that machine into a piece of electronics that takes it to the central counting place," Haley said. "That data is not encrypted and that's vulnerable for manipulation."

"How big of a hacking potential problem is this?" Villarreal asked him.

"Well, there's a huge potential," Haley responded. "There are so many places in the voting process once it goes electronic that's vulnerable."

According to a report from the Brennan Center for Justice, one reason these voting systems are so vulnerable is their age.

"We found that more than 40 states are using voting machines there that are at least 10 years" old, Brennan Center for Justice researcher Christopher Famighetti said.

Denise Merrill, president of the National Association of Secretaries of State, said the lack of funding keeps most precincts from updating their systems. But all machines have to meet specific government standards.

"The idea of a national hack of some sort is almost ridiculous because there is no national system," Merrill said.

In fact, the more than 9,000 voting districts across the country all have different ways of running their elections -- down to the type of machine they use. But Merrill said there are checks in place to prevent fraud.

"Our voting systems are heavily regulated. They're tested both before and after. There are paper trails everywhere...by in large, I would say the American election system works very well," Merrill said.

CBS News learned that only 60 percent of states routinely conduct audits post-election by checking paper trails. But not all states even have paper records, like in some parts of swing states Virginia and Pennsylvania, which experts say could be devastating.

The Election Assistance Commission told CBS News that it ensures all voting systems are vigorously tested against security standards and that systems certified by the EAC are not connected to the Internet.