Two U.S. senators say they are investigating short video-sharing app TikTok’s reported decision recently to hire several high-level executives from its Chinese parent company, ByteDance. The senators, Richard Blumenthal, a Democrat, and Republican Marsha Blackburn, said in a letter to TikTok CEO Shou Zi Chew that the hirings further call “into question the independence of TikTok’s operations and the security of its U.S. users’ information.”

The letter cited a Wall Street Journal report that found several former ByteDance engineers transferred to TikTok and took top roles in advertising, human resources, monetization, and business marketing. It also said some of those same execs oversee sensitive personal data. The senators want to know why the transfers were made, what positions the execs now hold, and whether any information was shared with the Committee on Foreign Investment in the United States before the Journal’s story.

ByteDance’s connection to TikTok has raised national security concerns, particularly about China spying on American citizens and companies. The firm admitted in December that some of its employees, including two based in Beijing, accessed the private data of U.S. journalists in an attempt to trace their I.P. addresses. The employees have since been fired, and the company says it has strengthened employee monitoring.

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Chew testified on Capitol Hill on Wednesday that TikTok stores the data of its 150 million American users, which is used to create and share videos, in servers maintained by software giant Oracle. He argued that data could only be accessed in the United States and Singapore, and it was backed up on computers located within the United States. He also said that any data transfers between the two countries require approval by a security team in the U.S.

NBC News reports that while most tech firms are cutting jobs and restraining spending, TikTok is continuing to pursue its plan to set up a team in Mountain View, California, to oversee the company’s data management. That team would determine who should have access to the company’s database and monitor their activity. It is a move meant to address concerns about the company’s reliance on staff in China and to ensure the company can meet any requirements from CFIUS, which must review any deal that gives ByteDance majority ownership of TikTok.

ByteDance is already facing a request by CFIUS to divest its investment in dating app Grindr. That’s partly because of concern that the app is being used to target women with rape-like messages. The company has denied those allegations and says the app is used to connect people with similar interests. It has also said it is working to remove offensive content from the platform. A spokeswoman for the Treasury Department, which chairs CFIUS, wouldn’t comment on specific cases. NBC’s Jo Ling Kent and Dylan Byers contributed to this report. NBC’s Jo Ling Kent is the business and technology correspondent, and Dylan Byers is a senior media reporter.