を調べている

is investigating

→ を
phrase Rare #22,085
is investigating Used when someone is currently investigating, examining, or looking into something. The を marks what is being investigated, and ている indicates an ongoing action.
JA: 私はその問題を調べている。
EN: I am investigating that problem.

警察はあの人たち経歴を調べているところだ

The police are investigating those people's backgrounds.

警察はその交通事故を調べている

The police are investigating the traffic accident now.

私たちを調べている

They are investigating us.

— 風の谷のナウシカ (1984)

警察は事故の原因を調べている

The police are investigating the cause of the accident.

警察はその交通事故の原因を調べている

The police are investigating the cause of the traffic accident.

警察がその事件を調べている

The police are investigating the case.

警察はその殺人事件を調べている

The police are investigating the murder.