United went into their final group fixture knowing that a victory would secure their progression to the knockout stages.
And it was a strong side that faced the Cod Army with first team squad members Taylor Charters, Louis Gray, Max Hunt, Jack Iredale, Jon Mellish, Olufela Olomola, Omari Patrick, Mo Sagaf and Stefan Scougall involved from the start.
United fashioned an early chance when Scougall and Charters combined to make space. Olomola took over with his back to goal but his shot on the turn was smothered and Crellin was able to gather it in.
The game stayed tight but a loose pass from Mellish brought Saunders into play. He spread it wide to Rydel but the cross from the Fleetwood man was too close to Gray and he was able to pull it into his chest.
United found a way beyond the back line when Sagaf overlapped on the left wing. He beat his man and rolled the ball into the area, and Olomola pounced to put his boot through the shot. Hill got himself in the way and the rebound hit the unlucky Carlisle forward before spinning behind for a goal kick.
Fleetwood took the lead with their first real chance of the day when a quick free kick saw Matete send the ball into the Carlisle half. Saunders was onto it, as Gray rushed out, and the striker somehow found the back of the net with an instinctive volley from what was a very tight angle.
The Cumbrians looked to respond and a one-touch passing move involving Lightfoot, Patrick and Charters left space for the latter to shoot. His effort from 23-yards had power but it rose too high.
Carlisle came again and a long punt forward was controlled well by Patrick. His second touch was a volley and it was heading for the bottom corner until it struck Rydel on the shin. That was enough to bend it away from the target.
United were back on terms just before the half hour when a lovely clip into the area was shielded by Olomola. He feinted to turn, but he spotted Patrick in space. A neat lay-off was followed by a crunching drive from the new arrival and that left Crellin with no chance.
There was more defending for the home side to do from the restart when Baggley pulled wide. His delivery was nodded goalwards by Saunders but Hunt was there to hook it away before it could cross the line.
Carlisle upped the pace ahead of the break and a deep corner from Charters was met well by Hunt. He sent his header back across the face of goal but it evaded the blue shirts and Smith gratefully smashed it away from danger.
And the hosts were close again when Iredale made room for a cross as he wound his way in from the left wing. It took a wicked ricochet and pinged towards Olomola, and he instinctively went for the volley. He was ready to celebrate but it curled its way just past the angle of post and bar.
It was a good spell for the Blues and Adam Walton had just one thing in mind as he arrived late to take a pass from Patrick. A rasping drive turned in the air and agonisingly swerved away from the corner of the net.
The second half took a while to get going but a cross from the right from Lightfoot caused the Fleetwood defence all kinds of problems. Patrick stabbed at it as it came his way and Crellin was relieved to see it creep past his upright.
There was danger at the other end when Smith sent a free kick deep. Holgate got there first, but his header was cleared as Hunt got himself in the way.
Smith was involved again when he strode forward with the ball at his feet. He tried his luck from distance but it skewed off the top of his boot and Dewhurst watched in comfort as it angled wide of the mark.
Carlisle were almost undone on the hour mark when a lob over the top sent Saunders through. Hunt recovered to make a last-ditch tackle to prevent the striker from bagging his second of the day.
That brought the hosts to life and a corner from Charters, following good work from Patrick, picked out Hunt in a good position. His header had power but it dropped just over the bar.
United should have taken the lead midway through the half when Charters shook off his man and cut the ball inside. Patrick took it into the box and looked up to see Olomola on the move. A rolled pass was trapped by the striker, but the moment of hesitation allowed Hill the time he needed to close the shot down.
It was suddenly end-to-end and Baggley rattled the woodwork with a rasping effort that struck the top of the bar before dropping onto the roof of the net.
Hill then went close for the visitors when he wrapped his boot around a volley from a Baggley corner. His effort bounced off Mellish and United were able to clear.
But disaster struck for Carlisle when a through ball was met by the dangerous Harvey Saunders. Very similar to his first half goal he found the back of the net from an ever-tightening angle.
A thunderbolt from Charters squirmed through the gloves of Crellin late in the day and he was relieved to see it fall at the far side of his post. Charters put the corner delivery into the box and the keeper redeemed himself as he made a two-handed diving catch fom an accurate Patrick header.
More good work from Charters saw him twist and turn before feeding it onwards for Olomola to shoot. Crellin threw a boot out to make another very good save.
The door stayed shut through time added on and the defeat means United must now wait to see what has happened with results elsewhere before their progression to the knockout rounds is either confirmed or denied.
Carlisle United - Patrick (29)
Fleetwood Town - Saunders (18, 73)
Carlisle United - Gray (Dewhurst 46), Lightfoot, Mellish, Hunt (c), Iredale, Charters, Scougall (Walton 12), Sagaf, Birch, Olomola (Wilson 90), Patrick (Leslie 81).
Fleetwood Town - Crellin, Goldsborough, Rydel, Smith, Hill, Holgate, Matete (Boyle 85), Garner, Saunders, Morris (Johnston 81), Baggley. Subs - Makepeace, Czajor, Patterson.
Referee - David Pill