After a little digging, here's some more factual info from the NTSB:
Braking action reports:
At 18

11 UA flt 1446 reported "Fair"
At 18:53:10 WN flt 2920 reported "Fair and it’s poor at the end here" [This is apparently the report that approach control was passing along as from a "company flt" - Jim]
At 19:01:33 WN flt 321 reported "Good first half of 31 Center, poor the second half" [Apparently the report that the tower controller passed along without reference to it being a "company flt" - Jim]
At 19:09:08 a C500 (Citation) reported "It’s poor right now (crew indicated they were passed taxiway A) [The Citation that the approach controller mentioned, but this apparently wasn't passed on to WN 1248 - Jim]
At 10

04 a G-IV reported "Fair to poor" [WN 1248 was on the frequency when this report was given - Jim]
Five FDR equipped transport aircraft landed in the 25 minutes prior to flt 1248. In order of their arrival, their brake use was:
UA 1446 used the Medium setting on autobrakes, which reached a maximum brake pressure of 448 psi 3.5 seconds after touchdown. Autobrakes were disengaged 11 seconds after touchdown when manual braking increased brake pressure.
The following aircraft were all WN flights and used manual braking:
WN 2920 reached full 3000 psi brake pressure within 6 seconds after touchdown.
WN 321 did not exceed 700 psi brake pressure until 20 seconds after touchdown when differential braking was used.
WN 2947 did not exceed 700 psi brake pressure until 14 seconds after touchdown when differential braking was used.
WN 1830 also used maximum brake pressure, but not until 19 seconds after touchdown as reverse thrust was decreasing.
The accident aircraft:
WN 1248 - the autobrakes activated 3.3 seconds after touchdown and reached maximum pressure of approximately 2500 psi (2400 for the left brake & 2600 for the right) approximately 8.3 seconds after touchdown. Autobrakes were disengaged 12 seconds after touchdown and brake pressure increased to 3000 psi. The aircraft left the runway 33.8 seconds after touchdown, at a speed of 39.5 kts, with T/R deployed and brake pressure at 3000 psi.
The FDR traces show that all these other aircraft achieved deceleration rates of approximately 0.3 gee, regardless of brake usage. Flight 1248 achieved a maximum of 0.24 gee.
Simulator recreations:
Approach 1: Full manual braking and reverse thrust at touchdown, WET-FAIR2 runway conditions, and touchdown at 1,500 feet.
Result: Able to come to a complete stop about 700 feet from end of runway. The on board performance computer (OPC) on the accident airplane calculated 560 feet of runway remaining after stop for this approach and these parameters.
Approach 2: Full manual braking and reverse thrust at touchdown, WET-POOR runway conditions, and touchdown at 1,500 feet.
Result: Able to come to a complete stop about 100 feet from end of runway. The OPC on the accident airplane calculated 50 feet of runway remaining after stop for this approach and these parameters.
Approach 3: Full manual braking and reverse thrust delayed 15 seconds after touchdown, WET-FAIR conditions, and touchdown at 1,800 feet.
Result: Able to stop about 100 feet from end of runway.
Approach 4: Full manual braking and reverse thrust delayed 15 seconds after touchdown, WET-POOR conditions, and touchdown at 1,800 feet.
Result: Airplane ran off the end of the runway.
Jim