addGestureRecognizerが設定されない

下記コードで、for文で15個のpiece(UIImageView)に対して、addGestureRecognizerでGestureを設定しているのですが、最後のpieceにだけGestureが設定されており、他には設定されまえせん。特にエラーも発生せず困っております。なぜでしょうか？

import UIKit

class ViewController: UIViewController {

    var board: UIView!
    var piece: UIImageView!

    enum Direction {
        case Left, Right, Up, Down, NG
    }

    let PieceSize: Int = 70
    var empty: Int = 15
    var panning : Bool = false
    var pieceX: CGFloat = 0
    var pieceY: CGFloat = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        board = UIView(frame: CGRectMake(0, 0, CGFloat(PieceSize * 4), CGFloat(PieceSize * 4)))
        board.center = view.center
        board.backgroundColor = UIColor.grayColor()
        board.layer.borderColor = UIColor.blackColor().CGColor
        board.layer.borderWidth = 1.0
        view.addSubview(board)

        let panGesture = UIPanGestureRecognizer(target: self, action: "pan:")

        for i in 0...14 {
            pieceX = CGFloat(i % 4 * PieceSize)

            if i < 4 {
                pieceY = CGFloat(0)
            } else if i > 3 && i < 8 {
                pieceY = CGFloat(PieceSize)
            } else if i > 7 && i < 12 {
                pieceY = CGFloat(PieceSize * 2)
            } else if i > 11 {
                pieceY = CGFloat(PieceSize * 3)
            }

            piece = UIImageView(frame: CGRectMake(pieceX, pieceY, CGFloat(PieceSize), CGFloat(PieceSize)))
            piece.image = UIImage(named: i.description + ".png")
            piece.userInteractionEnabled = true
            piece.tag = i
            piece.addGestureRecognizer(panGesture)
            board.addSubview(piece)
        }

        for view in board.subviews {
            print(view)
        }

    }

    func movePiece(aPiece: UIImageView) {
        if isMovable(aPiece.tag) != Direction.NG {

            // ピースを移動する
            var newFrame = CGRectMake(0, 0, CGFloat(PieceSize), CGFloat(PieceSize))
            newFrame.origin.x = CGFloat(PieceSize * (empty % 4))
            newFrame.origin.y = CGFloat(PieceSize * (empty / 4))
            aPiece.frame  = newFrame

            // 空白位置の入れ替え
            let oldEmpty = empty
            empty = aPiece.tag
            aPiece.tag = oldEmpty
        }
    }

    func pan(gesture: UIPanGestureRecognizer) {
        let aPiece = gesture.view as! UIImageView
        let direction = isMovable(aPiece.tag)

        if (direction != .NG) {
            if (gesture.state == UIGestureRecognizerState.Changed) || (gesture.state == UIGestureRecognizerState.Ended) {
            let translation = gesture.translationInView(aPiece)

            // 空白位置の方向に応じて動かす方向を限定する
            if (direction == .Left) && (translation.x < 0) {
                // 左に動かす
                panning = true
                aPiece.center.x = aPiece.center.x + translation.x

            } else if (direction == .Right) && (translation.x > 0) {
                // 右方向に動かす
                panning = true
                aPiece.center.x = aPiece.center.x + translation.x

            } else if (direction == .Up) && (translation.y < 0) {
                // 上方向に動かす
                panning = true
                aPiece.center.y = aPiece.center.y + translation.y

            } else if (direction == .Down) && (translation.y > 0) {
                // 下方向に動かす
                panning = true
                aPiece.center.y = aPiece.center.y + translation.y
            }

            // 移動距離をリセット
            gesture.setTranslation(CGPointZero, inView: aPiece)
            }
        }

        // パンが完了したらピースを移動
        if gesture.state == UIGestureRecognizerState.Ended {
            if panning {
            movePiece(aPiece)
            }
            panning = false
        }
    }

    func isMovable(tapPos: Int) -> Direction {
        var result: Direction = .NG

        if empty == tapPos + 4 {
            // 下に移動
            result = .Down
        } else if empty == tapPos - 4 {
            // 上に移動
            result = .Up
        } else if empty == (tapPos - 1) && (tapPos % 4) != 0 {
            // 左に移動
            result = .Left
        } else if empty == (tapPos + 1) && (tapPos % 4) != 3 {
            // 右に移動
            result = .Right
        }

        return result
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}