The Federal Aviation Administration has been increasing its scrutiny of Boeing’s plant near Charleston, S.C., where manufacturing errors have at times threatened to undermine safety.

Since September, the agency has investigated and confirmed three safety complaints made by employees who detailed problems with planes in the final stages of production, according to an F.A.A. official and an internal agency email. The regulator is also looking into a claim that an employee faced pressure to sign off on work related to the airworthiness of a jet during the last week of March.

In recent years, the F.A.A. has subjected the North Charleston factory to a high level of oversight, a sign that the agency has had concerns. As part of that added oversight, the agency has forced employees to take extra steps to demonstrate that they are complying with federal regulations, according to an internal F.A.A. memo reviewed by The New York Times.

From June 2013 to October 2014, the agency did not allow employees from the plant to certify aircraft, instead requiring that F.A.A. personnel directly sign off on all jets made there, the memo said. Typically, the agency relies on the manufacturer’s employees to help certify the aircraft. Even today, the agency is visiting the plant “every other week” to ensure that tools are not being lost, an extra level of vigilance by the regulator.