SEATTLE — Amazon has yet to find the key to unlocking huge grocery sales. Now, the company is trying a new tack: free delivery for more people.

On Tuesday the company announced free two-hour grocery delivery for Prime members in more than 20 major metropolitan areas. The changes eliminate what had been a monthly $14.99 fee for people shopping on Amazon Fresh, its online grocery service that delivers milk, eggs, seafood and other perishable products from its warehouses.

The new delivery option is in addition to the free delivery Amazon offers to Prime members who shop at most Whole Foods Market stores. Both methods require a minimum purchase for $35 to have no delivery fees. Prime memberships typically cost $119 a year.

Amazon has long wanted to make headway in the $700 billion grocery industry — it is not only an immense market, but also a category in which customers buy with high frequency.