According to The Wall Street Journal (WSJ), U.S. officials are finally disclosing the basis of their claims that Huawei poses a significant security risk.

U.S. officials have been claiming for some time that Huawei represent a fundamental security risk for network operators and their countries, opening them up to spying by Beijing. The U.S. has engaged in an aggressive campaign to pressure its allies to ban Huawei from their networks. In spite of this, the U.S. has never officially said what it based the accusations on—until now.

According to the report, U.S. officials say that Huawei is exploiting a legitimate backdoor that is reserved for law enforcement. Network equipment manufacturers are supposed to build backdoors in their equipment that carriers can use to grant access to law enforcement when required. Manufacturers, however, are supposed to build the backdoors in such a way that they are not able to access them, leaving only the carrier and law enforcement with access.

In Huawei’s case, however, U.S. officials claim the company has built the backdoors in its equipment in such a way that it maintains access, without the carriers being able to detect it.

“We have evidence that Huawei has the capability secretly to access sensitive and personal information in systems it maintains and sells around the world,” said Robert O’Brien, national security adviser.

The U.S. has known of this capability for at least a decade, but has kept the information strictly classified until late last year, when the information was shared with Germany and the U.K. With these new revelations, it remains to be seen if countries will start taking a stronger stance against the Chinese firm, as the U.S. has been campaigning for.