AMHERST — Earlier in the season, the University of Massachusetts was winning from beyond the arc, twice tying the school record for 3-pointers in a game. But now treys are their undoing.

The Minutemen once again were the gang that couldn’t shoot straight from long range (5 of 19) Tuesday night, and compounding their distant woes, they allowed George Washington’s top scorer a wide open 3-pointer early in the second half that sparked a flood of them as the Colonials broke away for an 81-70 victory at the Mullins Center.

It was the third straight Atlantic 10 defeat for UMass (8-8, 1-3).

The game was tight throughout the first half and into the second, until GW leader Tyler Cavanaugh sparked the Colonials (14-3, 3-1) by contributing in a whole new way. To that point, the junior forward already had 16 points, but mostly they had come on the inside and at the free throw line, where he was 8 of 10.

But 4½ minutes into the second half, Cavanaugh caught a pass at the top of the key and UMass defenders inexplicably backed away from him. He didn’t hesitate before launching a 3-pointer that gave GW its largest lead to that point, at 45-40. (He finished with a game-high 26 points.)

Then, on the other end, the ball ended up in the hands of Donte Clark (19 points) on the right wing, a good couple of steps beyond the arc. He didn’t hesitate, either, throwing up a trey. It was an airball, barely grazing the net. UMass coach Derek Kellogg immediately called a timeout.

It didn’t help. Matt Hart hit another 3-pointer for the Colonials, and while UMass did pick up the pace and pull within 3, GW continued hitting killing treys, with Alex Mitola (11 points, 7 assists) sinking a pair and Patricio Garino (17 points) beating the shot clock with one that made it a 61-52 game with 9:06 to go.

“That’s a good team,’’ said Kellogg. “They’re one of the better teams, obviously, in the league. But toe to toe, for a stretch there, we were right there with them.’’

His team’s undoing, said the coach, was not simply its outside shooting and its defense on the opponent’s long-range offense. “We’ve got to make better basketball plays and decisions in the open floor,’’ said Kellogg, whose team had just 4 points in transition and 15 turnovers. “We had a lot of opportunities, but we just didn’t convert. In years past, we converted at an 85-90 percent rate. Tonight, I thought our conversion rate was not very good. If you’re going to press and force tempo, you have to convert on the offensive end of the floor.’’

UMass was the more athletic team but never seemed to get in rhythm offensively. And while the home team’s defense was stout for long stretches, GW’s veteran lineup had the cunning to get the job done.

For example, the teams went into the locker rooms at intermission in a 34-34 tie after UMass held George Washington to one basket in the final seven minutes of the first half. The problem for the Minutemen was that they repeatedly sent the visitors to the line, and the Colonials made all 11 of their free throws during the stretch.

Kellogg didn’t like watching his team commit so many fouls, of course, but he liked the fight he saw in his team. “We’re going to be right up in your shirt,’’ he said. “We’re going to be a pressing team and play UMass basketball.’’