Deutsche Bank AG earlier this year flagged around $30 million in potentially suspicious transactions as part of an internal investigation into its role as a conduit for money involving Paul Manafort or people and entities connected to him, according to a person briefed on the matter.

The findings, which were discussed inside Deutsche Bank in late spring and early summer, primarily concerned cash flows tied to Mr. Manafort, who for about five months was President Donald Trump’s 2016 campaign chairman, and Mr. Manafort’s former...