TALLAHASSEE, Fla. — Mason Heintschel threw for 321 yards and connected with Desmond Reid on a pair of touchdown passes as Pittsburgh handed No. 25 Florida State its third straight conference loss on Saturday.

A true freshman, Heintschel completed 21 of 29 passes and had a pair of second-quarter interceptions in the 34-31 win. He has surpassed 300 passing yards in both of his starts, building off a rout of Boston College last week with a road upset of the Seminoles.

Reid had eight catches for 155 yards and 10 carries for 38 yards for Pittsburgh (4-2, 2-1 Atlantic Coast Conference).

The Panthers closed as 11.5-point underdogs before the game, making the victory their largest upset since beating No. 2 Miami in 2017 as 12-point underdogs, according to ESPN Research.

After a promising start to the season, Florida State (3-3, 0-3) is in free fall following losses to Virginia, Miami and now Pittsburgh. Mike Norvell’s team has failed to defeat an ACC opponent since a win over California more than a year ago.

Tommy Castellanos completed 16 of 23 passes for 245 yards for the Seminoles on Saturday, including a pair of touchdowns to Micahi Danzy — 58 yards in the fourth quarter and 33 yards in the second quarter.

But after FSU went ahead 24-21 on Jake Weinberg’s 34-yard field goal attempt, Florida State fumbled at midfield and then went three-and-out. Later, while trailing 34-24, came Castellanos’ 58-yarder to Danzy.

The Panthers finished with a pair of field goals and then Ja’Kyrian Turner’s 3-yard touchdown run capped an eight-play, 75-yard drive with 2:28 left.

The Associated Press contributed to this story.