a needle in a haystack

これは多分私自身がランニングをほぼ日課にしているからですが、大分前のニュース記事なのに事件のことを覚えています。

ニューヨークで美人ジョガーが絞殺されたという事件です。記事によれば事件は昨年8月でしたが、ついに犯人逮捕に至ったということです。


Finding Karina Vetrano’s killer was like searching for “a needle in a haystack,” an NYPD official said a day after a Brooklyn man was charged in the brutal murder of the beautiful Queens jogger.

“This case was very similar to a needle in a haystack because this individual had no prior arrests,” said Chief of Detectives Bob Boyce on Monday, referring to accused killer Chanel Lewis, 20.

During the six-month investigation into the high-profile Aug. 2 killing, investigators poured over 250 leads and 600 samples of DNA until the case finally saw a break last week, Boyce said.

Following up on a 911 call from the spring — and pushed by NYPD Lt. John Russo, whose relatives live near where Vetrano resided with her family — police were able to track down Lewis, who gave a swab of DNA to investigators.

Lewis’s DNA was linked to genetic material found under Vetrano’s fingernails, on her back and on her backpack, authorities have said. He later confessed to strangling the runner, sources said.
(Jogger suspect's clean record made finding him even harder. New York Post. February 6, 2017.)


犯人逮捕にここまで時間がかかったひとつの理由として、犯人には前科が無かったことが挙げられています。そして、そのことは、"look for a needle in a haystack"（干し草の山から1本の針を探すようなもの）だった、と表現されています。

"look for a needle in a haystack"は成句で、とても無理なことの喩えです。

日本語だと砂漠の中から砂金を探す、でしょうか。