EAST LANSING, Mich. — The Michigan State student section was a solid, standing, swaying sea of Spartan green in the middle of the third quarter of Saturday’s game. Eight minutes later the sea had departed and only tiny drops of green dotted the metal bleachers.

Michigan State students knew the game had been decided, and they headed home early to start preparing for a night of celebration.

“The story of the game came down to the third quarter,” Northwestern coach Randy Walker said. “We had a great opportunity to take the momentum and we let it slip away.”

The Wildcats stayed close to the Spartans in the first half of their Big Ten opener and had a chance to take the lead — and steal momentum — in the third quarter, trailing 20-17. But NU (2-3, 0-1 Big Ten) failed to score on two consecutive promising drives, and the Spartans (3-2, 1-0), who rolled up 276 yards rushing, cruised to a 39-24 victory in front of 74,215 at Spartan Stadium.

After two Michigan State scores extended the lead to 20-10 at the half, NU cornerback Raheem Covington halted a Spartans scoring drive by intercepting a pass in the endzone intended for Heisman candidate Charles Rogers.

The Cats offense responded by using the no-huddle to go 80 yards in seven plays, ending with a touchdown run by running back Noah Herron.

On the ensuing kickoff, the Cats caught Michigan State off-guard with a high chip-shot onsides kick that bounced between Spartans and was recovered by NU freshman running back Terrell Jordan.

“The kickoff recovery was a big momentum swinger, and if we could have punched it in the endzone it would have been a different ball game,” said running back Jason Wright, who had 134 yards rushing on 22 carries. “But there were a lot of ifs in there.”

The Cats couldn’t capitalize on the possession, losing one yard on three plays. They walked away empty-handed as kicker David Wasielewski’s 50-yard field-goal attempt sailed wide right.

But Covington, who held Rogers to just three catches for 53 yards, gave the offense another chance when he stepped in front of the preseason All-American and picked off his second pass in as many series.

The offense again failed to move the ball and Michigan State punt returner Ziehl Kavanaght took Brian Huffman’s punt back 88 yards untouched to the endzone to extend the Spartans’ lead to 10.

“Our offense had its hands on the ball with a chance to really seize the momentum,” Walker said, “and our special teams let us down, and before you knew it, it was an explosion.”

NU never recovered from the punt return as Michigan State scored 19 straight points. After the return, the Spartans put nine quick points on the board.

When NU tried to answer the Spartans’ run, a turnover at the NU 31-yard line put its defense in a hole. Wide receiver Ronnie Foster was upended after catching a screen pass, and the ball came loose. The officials ruled it a fumble, despite hearing an earful from Walker who rushed onto the field.

“I thought we got a little unsettled out there, including me probably,” Walker said. “I was out there yelling about stuff and getting mad. And one of the first things I did was apologize to the team.”

The Cats scored only once more after Walker’s sideline tirade. Sophomore quarterback Tony Stauss replaced starter Brett Basanez for the final two series, and in his third play under center hit tight end Eric Worley on a 48-yard touchdown strike to cap a three-play, 80-yard drive.

Stauss provided a lift to an offense that generated only 136 yards — 80 on one touchdown drive — in the second half before he entered the game.

“Everyone on the offense played well, but we just can’t have those letdowns,” Basanez said. ” The third quarter killed us, and we can’t have that going on in our play. We just need to execute.”

In the first half, NU’s offense ran a balanced attack, responding to two Michigan State scores with scores of its own. Basanez threw for 108 yards and completed passes to four different receivers, including Jon Schweighardt, who made a diving catch on a eight-yard touchdown pass.

At tailback, Wright scampered for 110 yards on 16 carries in the first half before being held to only 24 yards after halftime.

“We were clicking pretty well in the first half, but we still could have been more efficient,” Wright said. “In the first half we scored 10 points, and we should be scoring more than that.”

The Spartans had no problem scoring, but it was not with their traditional offense.

Covington and the Cats shut down Rogers — except for his touchdown catch when he was sandwiched between defenders and managed to reach behind his body to steal the ball. Without Rogers carrying the offense, the Spartans turned to a ground attack that piled up 276 yards.

NU knows it needs to improve against the running game, and the team’s veterans know they let a potential victory slip away.

“We’re not going to feel good about playing Michigan State tough,” said Schweighardt, one of four senior starters. “We should have won this game. That’s the way we’re going to look at it. We know we can play with them. We had an opportunity to win the game, and we just didn’t come through. We’re going to need to cash in our opportunities.”