Federico Higuain scored late in the first half and Kamara converted midway through the second before an announced 14,031 at rainy RFK Stadium.

United playmaker Luciano Acosta made his season debut after recovering from an ankle injury. He crackled in the first half, then faded in the second and left with leg cramps. Striker Patrick Mullins departed in the 28th minute with a hamstring injury.

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Crew goalkeeper Zack Steffen, a former University of Maryland standout, made two sterling saves in the second half, thwarting a United attack that late last season was among the best in MLS.

“I didn’t think it was a terrible performance. Overall, it was a step in the right direction,” Coach Ben Olsen said a week after a 4-0 defeat at New York City FC. “Some good performances, but not enough good performances.”

The two penalty-kick decisions were the difference, but with United’s attack sputtering, one would’ve been enough.

“First one I think is probably a good call and the second one’s a terrible call,” Olsen said. “I’m pretty sure on both of those. Referees should be pretty sure on calling PKs.”

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Birnbaum had a similar view.

“I thought I got a touch [on the ball] on the first one, but I came across him and definitely touched him. Second one, I thought was pretty good defense. I guess you can’t touch him in the box. Obviously, it cost us the game, both of them.”

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United was encouraged at the start. With rookie Ian Harkes providing support, Acosta influenced the match with his darting runs, quick footwork and accurate distribution. But still no goals.

Bill Hamid bailed out United in the early stages, thwarting Higuain twice and blocking Justin Meram’s smash.

The Crew (1-1-1) broke through in the 38th minute on Higuain’s penalty kick.

Four minutes into the second half, United could have pulled even with wonderful combination work. Jose Guillermo Ortiz, Mullins’s replacement, played the ball to Lloyd Sam, whose back heel from the top of the box led Ortiz rushing the right side. However, Steffen came off his line and blocked the rising bid.

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Still no goal.

Birnbaum ran down a high ball but, from an acute but manageable angle, headed it well wide.

Still no goal.

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Olsen turned to Sebastian Le Toux in the 60th minute, the Frenchman’s first appearance since joining the club this season.

Still no goal.

Midway through the half, Kamara collected the ball in the box and took on Birnbaum. Birnbaum put his hand on the striker’s hip and appeared to push. It wasn’t an egregious act, but enough to catch Unkel’s attention and result in another penalty. Hamid guessed correctly again but couldn’t stop Kamara’s low effort.

It was the end to a rough week for Birnbaum, who didn’t perform well last weekend and was a surprise omission from the U.S. national team roster for the two upcoming World Cup qualifiers.

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Steffen preserved the shutout with an outstanding save on Nyarko’s 12-yard header in the 70th minute.

Still no goal.

Acosta’s cramps opened the door for Lamar Neagle, last year’s goal leader.

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Still no goal.

“It’s frustrating,” defender Taylor Kemp said. “I do think this was a better performance from us as a group. Our possession was better, our movement was a little bit better. Clearly we’ve got to get better in the final third.”

With next weekend off, United will have to wait until April 1 against visiting Philadelphia to end the scoring drought.