Subway co-founder Peter Buck, who turned his $1,000 investment into a multibillion-dollar franchise, dies aged 90

• Subway's last surviving co-founder, Peter Buck, died on Thursday at 90 years old.
• Buck initially invested $1,000 to help Fred DeLuca start the submarine sandwich shop in 1965.

Subway's last surviving co-founder Peter Buck passed away at Danbury Hospital in Connecticut on Thursday, according to a company statement.

In a statement to CNN, John Chidsey, Subway's CEO, said: "We are deeply saddened by the passing of one of Subway's founders, Dr. Peter Buck. He was a shining example of a dedicated, hands-on leader, and an integral member of the Subway family."

Buck was 90 years old. His cause of death has not yet been revealed.

The nuclear physicist-turned-entrepreneur, was 34 when he gave 17-year-old Fred DeLuca, a family friend, the initial $1,000 to start a submarine sandwich shop, CNN reported.

Buck gave DeLuca the idea at a family picnic, according to Forbes. The outlet quoted Buck as saying: "At some point during the picnic, Fred asked me what he could do to make some money so he could go to college. I said, let's open a submarine sandwich store."

The restaurant opened in 1965 in Bridgewater, Connecticut, and was originally called "Pete's Super Submarines" after its sole investor.

In 1974, after the pair had opened 16 more sandwich shops in the Connecticut area, they decided to start franchising them, according to a post on Subway's website.

Now Subway is the world's largest submarine sandwich chain, with 44,000 locations worldwide, its website states.

CNN reported that Subway's sales have declined in recent years, and are down nearly $4 billion from the record $12.3 billion sales it reported in 2013.

DeLuca died in 2015 at the age of 65, leaving Buck as the last surviving co-founder.

Buck's net worth was $1.7 billion and he was also the seventh largest landowner in the US, according to Forbes and The Land Report.