The massive, multi-agency search for two firefighters who didn't return from a fishing trip off the coast of Florida last week was suspended Thursday, according to the U.S. Coast Guard.

Brian McCluney, a firefighter with the Jacksonville Fire and Rescue Department, and Justin Walker, a firefighter with the Fairfax, Virginia, fire department, didn't return home Friday as expected after taking a 24-foot fishing boat out near Port Canaveral.

"Our Coast Guard men and women worked tirelessly alongside federal, state and local partner agencies as well as countless volunteers searching for Brian and Justin," Capt. Mark Vlaun, commander of Coast Guard Sector Jacksonville, said in a statement.

More than 20 agencies, including the Coast Guard, the Navy, U.S. Customs and Border Protection Air and Marine Operations, the FBI, the U.S. Marshals Service and the Florida Fish and Wildlife Conservation Commission were involved in the search for the two men. Volunteers, including teams of firefighters up the eastern seaboard, were also searching on the land, from the air, and in the water.

Brian McCluney and Justin Walker. via WESH

The Coast Guard said Thursday evening that more than 146,368 square miles, or an area equivalent to the size of Montana, had been searched.

Vlaun said during a news conference earlier Thursday that the search field spanned from Central Florida to New England, a vast area that was "no longer allowing us to search with any reasonable level of degree of success."

"In my 25-year career, I have never seen a more comprehensive or a larger search operation," Vlaun said. "We could not have put more into this than we did."

Vlaun said he explained to McCluney and Walker's families that "suspending a search does not stop global Coast Guard operations."

"Everybody's aware that we’re still looking for Justin and Brian," and crews returning to their normal posts might have a "better opportunity of of learning something more," at which point the search could be reopened," Vlaun said.

Crews were initially optimistic that the men, who have been friends since college, knew the area and fished often, would be found safe considering their line of work and McCluney's experience in the Navy.

But as the week went on, Vlaun increasingly referred to the search as a "race against time" while crews struggled to find anything that would narrow the search area down.

McCluney's tackle bag was found Monday by a volunteer 50 miles off the coast of St. Augustine, which is more than 100 miles north of Port Canaveral. Responders zeroed in on the area, but by Thursday, no other debris that could definitely be connected to the missing boat could be found.

Jacksonville Fire and Rescue Department Chief Keith Powers said calling off the search was "an extremely tough decision because we have a brother out there that we haven't been able to find."

He said the firefighters' families are "heartbroken." Walker is married, and McCluney is married with two kids.

He said donations made to fund the search topped $150,000, and anything left over will be used to support Walker and McCluney's families.