HATTIESBURG, Miss. — Taylor Lamb threw what appeared to be a game-tying two-yard touchdown pass to Simms McElfresh with six seconds to go but the ensuing extra point was blocked and Appalachian State University football suffered a heartbreaking 21-20 defeat at Southern Miss on Saturday night at M.M. Roberts Stadium.

Lamb, who was making his first-career start, engineered two touchdown drives in the final eight minutes of the game, including a 12-play, 75-yarder that was capped by McElfresh's over-the-shoulder catch in the back corner of the end zone that looked like it would send the contest to overtime. However, Southern Miss' Dasman McCullum blocked the extra point to maintain the Golden Eagles' one-point lead.

Southern Miss (2-2) recovered the ensuing onside kick and knelt out the clock to escape with the victory.

Appalachian State (1-2) dominated USM in most statistical categories, including total yards (455-329), rushing yards (191-61), first downs (29-16), third/fourth-down conversions (8-of-16 to 2-of-11) and time of possession (35:51-24:09). However, the Mountaineers had three turnovers, came away with just three scores on five trips in the red zone, committed 11 penalties and, most glaringly, left 10 points on the field in the kicking game. In addition to the block extra point with six seconds to go, App State also missed all three of its field-goal attempts.

Southern Miss jumped out to a 14-0 lead less than two minutes into the second quarter but Appalachian State's defense limited the Golden Eagles to just 101 total yards and 2.9 yards per play over their next eight possessions. The Mountaineers forced six three-and-outs during the eight-series stretch.

The dominant defensive performance allowed App State to get back into the game. The Mountaineers got on the board midway through the second quarter on a five-yard touchdown run by Marcus Cox and tied the game at 14-14 with 5:08 to go in the game on a 10-yard pass from Lamb to Malachi Jones.

However, following Jones' game-tying touchdown catch, Southern Miss responded with a six-play, 65-yard drive and retook a 21-14 lead on George Payne's third touchdown of the evening, a 31-yard reception from Nick Mullens, with 2:22 remaining.

On the next series, Lamb needed just 2:16 to move the Mountaineers 75 yards for the would-be game-tying score.

The final drive capped a terrific starting debut for Lamb. The redshirt freshman completed 28-of-40 passes for 264 yards and two touchdowns and ran eight times for 39 yards.

McElfresh and Jones were Lamb's favorite targets, as they both recorded seven receptions and a touchdown catch. McElfresh finished with 68 yards while Jones tallied 61.

Cox ran for a game-high 119 yards on 22 carries (5.4 avg.).

Appalachian State's outstanding defensive performance was led by inside linebacker John Law, who recorded eight tackles, including 2.5 for loss and a sack. Safety Doug Middleton added seven stops and cornerback Joel Ross had two interceptions.

The Mountaineers do not have much time to shake off the tough loss, as they open Sun Belt Conference play in just five days with a date at longtime rival Georgia Southern (2-2). The Sun Belt opener is set for 7:30 p.m. Thursday at Paulson Stadium in Statesboro, Ga. It will be televised nationally on ESPNU.

NOTES: Appalachian State is 1-2 for the third-consecutive season ... the Mountaineers lost a regular-season game that immediately followed a bye for the first time since a 35-32 defeat at James Madison on Sept. 20, 2008 ... Appalachian had won four-straight regular-season games that immediately followed a bye ... Appalachian State missed three field goals in a game for the first time since it missed three in a 28-14 loss at Wofford on Oct. 1, 2011 ... the Mountaineers lost on a blocked PAT for the second time in their last 16 games ... App State lost in similar fashion when Illinois State blocked an extra point in overtime to hand the Mountaineers a 38-37 defeat in the second round of the 2012 NCAA Division I FCS playoffs ... Cox's 100-yard rushing game was the eighth of his 15-game career ... Cox surpassed the 1,500-yard mark for his career in the first half ... in15 career games, the sophomore Doak Walker Award candidate has amassed 1,530 rushing yards (102 yards per game).