Connor Halliday threw for 383 yards and five touchdowns and Washington State piled up 464 yards against Southern Utah, an FCS team.
"This is the best day we've had in Leach's offense," said Halliday, who completed 32 of 41 passes and was intercepted once.
Ten different receivers caught balls.
"It was easy to get them the ball," Halliday said. "Everybody was wide open."
Washington State (2-1) also got a long interception return for a touchdown from Damante Horton for the second consecutive game.
"Picks come in bunches," Horton said.
Southern Utah (2-1) was scoreless in the second half and gained just 219 yards in the game.
"I was really impressed with the way their offensive line held out and their quarterback was able to sit in there and really slice us up," SUU coach Ed Lamb said.
Dom Williams caught three passes for 101 yards and two touchdowns for WSU. Gabe Marks had seven catches for 83 yards.
"Everybody got catches and it's fun to win," Williams said.
Aaron Cantu completed 22 of 30 passes for Southern Utah, but was intercepted three times.
"With the score as it was, we had to throw it a little more than we wanted to," Lamb said.
Washington State was coming off a 10-7 win at Southern California that knocked the Trojans from the Top 25.
Southern Utah beat an FBS team, South Alabama, in its season opener, but had less success against Washington State of the Pacific-12.
This was the first meeting between the teams.
Washington State completed passes on its first five plays of the game, the fifth covering 43 yards from Halliday to Williams for a touchdown and a 7-0 lead less than 2 minutes into the contest.
Southern Utah replied with a drive that stalled on the 23, leading to a 40-yard field goal by Colton Cook.
Washington State struck right back with a 61-yard drive that required just five plays, with Isiah Myers catching a 10-yard touchdown pass from Halliday for a 14-3 lead.
Halliday completed his first 10 passes of the game.
After WSU was forced to punt from its own end zone, Southern Utah took possession on WSU's 29. Raysean Martin punched over from the 3-yard line as the Thunderbirds cut WSU's lead to 14-10.
But the Thunderbirds would not score again.
"Defensively we heated up in the second half," Leach said.
Washington State marched down to the 19 on its next possession, but Halliday was intercepted in the end zone by a diving LeShaun Sims to kill the drive. Then DaQuawn Brown intercepted Cantu to give the ball back to WSU at midfield. Three plays later, Halliday fired a 55-yard touchdown bomb to Dom Williams for a 21-10 lead.
On Southern Utah's next drive, Cantu was intercepted at the 28 by Horton, who returned the ball untouched for 72 yards and a 28-10 WSU lead that stood at halftime. Horton's 70-yard interception return against Southern Cal last weekend was WSU's only touchdown of that game.
Andrew Furney kicked field goals of 46 and 30 yards in the third to give WSU a 34-10 lead.
Deone Bucannon picked off a Cantu pass and returned it to SUU's 18. Halliday threw a 1-yard touchdown pass to Gabe Marks in the end zone for a 41-10 lead.
Halliday added a 4-yard touchdown pass to Vince Mayle in the fourth.
Leach said this game will only help his team improve as the season progresses.
"Our receivers improved, Connor improved, our offensive line improved," Leach said. "This will help us get better."
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