(100nl) AK, flop top pair, bet it down, he jams on riv, what gives?

[FF_Woodycooks]

Villain is unknown.

Full Tilt No-Limit Hold'em, $1 BB (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

MP1 ($69.80)
MP2 ($50.70)
F.F. ($134.85)
CO ($117.95)
Button ($93.95)
SB ($33.25)
BB ($19)
UTG ($58.55)
UTG+1 ($87.25)

Preflop: F.F. is MP3 with A[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/diamond.gif[/img].
<font color="#666666">1 fold</font>, UTG+1 calls $1, <font color="#666666">2 folds</font>, <font color="#CC3333">F.F. raises to $5</font>, <font color="#666666">4 folds</font>, UTG+1 calls $4.

Flop: ($11.50) 2[img]/images/graemlins/spade.gif[/img], J[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
UTG+1 checks, <font color="#CC3333">F.F. bets $7</font>, UTG+1 calls $7.

Turn: ($25.50) 5[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
UTG+1 checks, <font color="#CC3333">F.F. bets $16</font>, UTG+1 calls $16.

River: ($57.50) 4[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">UTG+1 bets $59.25 (All-In)</font>, F.F. ?

[SABR42]

I call.

He either has A3 or is bluffing.