Perimeter shooting was a strength for BYU earlier in the season.
Now it has become a weakness for the Cougars.
St. Mary's experienced no such trouble knocking down shots from the outside.
It proved to be the difference for the Gaels in earning an 80-66 victory over BYU on Saturday.
St. Mary's shot 7 of 18 from the perimeter while limiting the Cougars to just 2 of 13 from the 3-point line.
It is part of a prolonged drought from the perimeter for BYU.
This time, it cost the Cougars a shot at a victory that could have greatly enhanced their NCAA tourney profile going into March.
"We just need to knock them down," freshman point guard Matt Carlino said. "Sometimes teams go through droughts. Ours is coming in a tough spot."
Brad Waldow scored 19 points and Clint Steindl added 16 off the bench for the Gaels (21-2, 10-0), who took control early on both ends of the court to remain atop the WCC.
Rob Jones also added 13 points and nine rebounds to offset leading scorer Matthew Dellavedova, who was held to just 10 points.
Steindl scored 14 points in the first half when he hit his first four 3-point shots. He finished 4 of 6 from behind the arc.
The Cougars (18-6, 6-3) were led by Noah Hartsock and Carlino, who each scored 15 points. Brandon Davies added 14 points and seven rebounds.
St. Mary's used a 10-0 run to build a 40-28 halftime lead, and stretched the margin to 23 points early in the second half.
BYU cut the lead to nine, but could never get closer.
"We were prepared and we were ready to fight the storm," Waldow said.
The Gaels held off the Cougars in the second half in part because of a rugged defensive effort.
BYU struggled to mount a serious run because St. Mary's forced the Cougars to rush shots and kept things under control even when possessing the ball.
"It seemed like we couldn't run anything," St. Mary's coach Randy Bennett said. "We just had to hang onto the ball. What kept us in it is I thought we did a really good job of staying with what we were trying to do defensively."
St. Mary's and BYU spent most of the first half using their inside game.
The difference in the opening 20 minutes, however, was Steindl. With the game tied 13-13, Steindl came off the bench to hit his first 3 from the right corner to give the Gaels the lead.
The Cougars got back to 22-20 on a Hartsock score, but Waldow then broke loose for seven points in the paint and another Steindl 3 pushed St. Mary's to a 32-20 lead.
"St. Mary's played terrific, especially in the first half, by spreading us out and having multiple players make big shots and big plays," BYU coach Dave Rose said. "They got us to where we were rushing more like we did when we played over there."
After taking a 12-point lead into the break, St. Mary's widened the margin to 55-32 at the 16:49 mark of the second half when Dellavedova and Jorden Page each hit a 3 and Waldow added two more baskets in the paint.
The Cougars went on a 15-5 run to cut the St. Mary's lead to 60-47, but the teams traded baskets for the next several minutes to keep the Gaels' margin at 13.
Carlino scored five straight to make it 70-61.
St. Mary's was able to hang on, however, when BYU turned the ball over on two straight possessions and the Gaels hit 8 of 10 free throws down the stretch to finish things off.
"We just tried to make them make tough shots," Bennett said. "They're going to score and they're going to get the ball (inside) and you can't keep that from happening. So we just wanted to limit the number of easy baskets they got."
The Gaels shot 56 percent from the floor to only 41 percent for the Cougars.
St. Mary's also made 25 of 33 from the line. The Cougars were 14 of 22 from the line. St. Mary's won despite turning the ball over 24 times.