Taiwan is investigating four Taiwanese companies for potentially violating US sanctions and its own investment rules by providing services to Chinese companies involved in building chip factories for Huawei. The companies, including Huawei, have been accused of illegally exporting semiconductor materials and equipment to China. The companies deny the allegations and claim they were only involved in approved activities. Taiwanese companies are closely integrated into China's supply chains but the government does not allow its most advanced technology to be produced there. The investigation comes amid increasing military pressure from China and as Taiwan's presidential election approaches.