A large transfer raised suspicions at the bank, prompting it to alert U.K. authorities at least weeks ago, the person said, asking not to be identified discussing confidential information. Britain’s National Crime Agency told Reuters last week it’s preparing to return funds to the oil-rich African nation.

 The seizure is being hailed by HSBC executives as a sign that its compliance systems work, the Financial Times wrote in a report earlier Tuesday. In 2012, the London-based lender paid a then-record $1.9 billion to settle U.S. claims it had helped drug cartels launder money and handled other illicit payments.

Jose Filomeno dos Santos, the son of Angola’s former president, said in a statement this week that he will cooperate with authorities after prosecutors accused him of fraud over the alleged transfer of $500 million from government coffers to the London branch of a Swiss bank. He had led the nation’s sovereign wealth fund.

The account at HSBC wasn’t in his name, the person familiar with the situation said. Instead it had been brought to the firm under the name of a company with no obvious link to Angola, the person said. Credit Suisse Group AG has discovered that documents purporting to be from the Swiss bank were used in the alleged fraud, but that the firm didn’t receive any related funds, according to the FT.

Dos Santos said he learned of his indictment and “terms of coercion” through the media on Monday, then walked to the state prosecutor’s office to hand over his passport as requested. The case also targets nine other people, local MFM Radio reported Monday.