It's the biggest threat facing American business today but the least talked about by corporate executives. Experts at the highest levels of government agree, cyber espionage is threatening to steal American wealth, American jobs and ultimately America's economic security and the biggest aggressor is China. Due to the nature of the crime, the cost to American businesses is nearly impossible to pinpoint. Experts say Chinese hackers are constantly probing corporate networks, sifting through endless amounts of data to decipher what is valuable intellectual property, chemical formulas or proprietary technology. One conservative estimate from the National Counter Intelligence Executive puts the cost of economic espionage at up to $400B annually, but the report states such estimates vary “so widely as to be meaningless,” reflecting the scarcity of data available. CNBC's David Faber and the Investigations Inc. team spoke with many corporate executives about China's aggressive effort to target American businesses and their most valuable assets, but many refused to comment on camera for our report, citing becoming more vulnerable to attack by speaking publicly about the issue. However, not one executive denied their company is at risk of cyber-attack on a daily basis or the possibility of losing valuable intellectual property to cyber spies. Government and industry experts we spoke with on-camera have witnessed such costly cyber-attacks during their careers and attest to the fact there are only two companies left in America today: Those who know they've been hacked and those who don't. From a whistleblower claiming telecommunications giant Nortel was one of the first casualties of this all-out cyber war, to high profile and public attacks on Google and RSA, its clear defending against cyber espionage is the new normal for American business.