Authorities have suspended the search for a missing uncle and nephew whose boat was found empty.

Days after Randall Spivey, 57, and his nephew Brandon Billmaier, 33, vanished, the U.S. Coast Guard said that the search for the men would be suspended “at sunset” on Monday, Dec. 22.

The Coast Guard said their officials and other partner agencies had searched for the men by air and sea, spanning about 6,700 square miles. The agency added that on-scene weather conditions during the operation consisted of approximately 10 mph winds and 3-foot seas.

“The Coast Guard diligently searched with our pilots, boat forces, cutter crews, and numerous partners,” said Lt. Amy Harrison, search and rescue mission coordinator for Coast Guard Sector St. Petersburg, in a statement. “We saturated an incredibly large search area but, unfortunately, were unable to locate the two missing men.”

One day after the men disapperead, the Lee County Sheriff's Office launched an urgent search on Saturday, Dec. 20, for the “endangered adults.”

“The two left early yesterday morning [Dec. 19] for a fishing trip on a 42FT Freeman boat from a residence on the 15000 block of Intracoastal Ct in Fort Myers,” the LCSO previously shared on Facebook.

The Coast Guard previously told PEOPLE that the agency was “working day and night with the goal of locating the missing boaters and returning them home to their loved ones.”

Billmaier’s wife, Deborah, announced a monetary reward on Instagram for anyone who found the two men.

“I am heartbroken posting this. My husband, Brandon, went out fishing yesterday with his uncle off the coast of Fort Myers,” she wrote in the post’s caption. “They left early in the morning on Friday, Dec. 19 and were meant to come back that afternoon.”

In its most recent news release, the Coast Guard said that its Air Station Clearwater helicopter crew found the boat adrift about 70 miles west of Fort Myers at approximately 12 a.m., on Saturday. The vessel was later brought back to Station Fort Myers Beach for additional investigation.

After careful review of the factors in the case, the agency said on Monday that it made the “difficult decision” to call off the search.

“There is no harder decision than suspending a search,” said Capt. Corrie Sergent, commanding officer of Coast Guard Sector St. Petersburg, in a statement. “I am incredibly grateful for the crews, partners, and volunteers across this community who executed this massive search with the utmost professionalism, persistence, and compassion. We offer our sincere condolences to the families and friends of Mr. Spivey and Mr. Billmaier during this extremely difficult time.”

In a Monday interview with ABC affiliate WWSB, Spivey's son, also named Randall, remembered receiving a phone call from the Coast Guard telling him that only their boat had been found.

“It was the worst few hours of my life and the worst phone call of my life,” he said. “I know my mom would agree if you could have heard how she reacted to that. I wouldn’t wish that feeling on anybody. It’s pretty indescribable.”

The Coast Guard stated that the FBI is leading the missing persons investigation.