Mitchell's long 3-pointer with 3.9 seconds capped a wild finish and gave the fourth-seeded Tigers a 58-57 victory over No. 12 seed Texas A&M in the second round of the Atlanta Regional on Saturday.
Glen Davis scored 21 points, including a key basket in the final minute. But it was Mitchell - LSU's lone senior starter - who saved the day after Texas A&M snapped a 55-55 tie with Acie Law's jumper over Mitchell's outstretched arms with 19.3 seconds left.
The Southeastern Conference regular-season champions seemingly were in control when Mitchell made two foul shots for a 53-46 lead with 3:31 to go. But Texas A&M (22-9), in the tournament for the first time since 1987, wasn't ready to go home.
The Aggies wiped out the seven-point deficit over the next 91 seconds, then went ahead 55-53 on Joseph Jones' jump hook in the lane.
Davis, the 6-foot-9, 310-pound SEC player of the year who also had eight rebounds, powered his way to the basket for a layup that tied it for the last time.
The victory sends LSU (25-8) to the regional semifinals against top-seed Duke (32-3) on Thursday night.
Mitchell was 1-for-7 from behind the 3-point line, and LSU was 1-for-11 as a team, before he seemingly launched the 22-footer in desperation. LSU's bench celebrated, but there was still time left -- especially with the speedy Law on the court for Texas A&M.
Law had 23 points, 12 of them in the final 1:25, in the Aggies' first-round victory over Syracuse. But this time, LSU did a good job of keeping the ball out of his hands and Texas A&M didn't get the ball past halfcourt on its final possession.
Law led the Aggies this time with 15 points. Antanas Kavaliauskas added 12. Mitchell joined Davis in double figures for LSU with 16.
Davis made his first two shots Saturday, helping LSU get off to a 15-3 lead. But the Aggies made him work for everything he got.
Texas A&M overcame its slow start with a 17-0 run, with Dominique Kirk making a pair of 3-pointers and Law also scoring six to lead the surge. It was 29-29 at the half after the 6-10 Kavaliauskas rose above the rim to catch Law's errant 3-point shot and lay the ball in at the buzzer.
The Aggies were in the second round of the NCAA tournament for only the third time, and their victory over Syracuse was their first in the NCAAs since their only trip to a regional semifinal in 1980.
Despite the loss, the Aggies clearly have a program on the rise under coach Billy Gillispie, the former Bill Self assistant who inherited a team that was winless in the Big 12 and won seven games overall two years ago.
LSU's run to the round of 16, meanwhile, is something the Tigers felt they needed to put a stamp on the climb they made in the SEC over the last two seasons. It's their first trip to the regional semifinals since 2000.
Brady's Bunch moves on in dramatic fashion
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