をじっと見つめていた

was staring intently at …

→ を
phrase Rare #40,698
was staring intently at … を marks the thing being stared at; the subject is understood from context.
JA: 彼は窓の外をじっと見つめていた。
EN: He was staring intently out the window.

毎朝朝食をとっている間、彼の犬は彼をじっと見つめていた。

Every morning, while he ate breakfast, his dog would stare intently at him.

その先生はしばらくの間私をじっと見つめていた。

That teacher stared at me for a while.

彼女は少女の美しい指輪をじっと見つめていた。

She stared intently at the girl's beautiful ring.