US investigators said that bombing suspect Ahmad Khan Rahami passed secondary screening at customs when he returned to the USA from trips to Afghanistan and Pakistan.
Officials were looking for clues as to why Rahami may have planted bombs in the New York area, including whether he was radicalised overseas or had accomplices.
Union County prosecutors charged Rahami with five counts of attempted murder in the first degree and two second-degree weapons charges after he was detained following a shootout with police on Monday.
He was in critical but stable condition as a result of his wounds but has not been interviewed in depth, New York Police Department Commissioner James O'Neill said on Tuesday.
More charges are expected to be brought against him in federal court after what New York City Mayor Bill de Blasio called "an act of terror".
Authorities are investigating whether Ahmad Rahami, who was not on US counter-terrorism lists, acted alone or as part of a terror cell.
Investigators studying CCTV footage may have identified the same man at each location, law enforcement sources reported to have said.
At least 29 people have been injured in an explosion which rocked Manhattan. Here's what we know so far.