US Homeland Security Secretary Janet Napolitano was in Doha on Sunday where she signed a bilateral aviation security agreement with senior Qatari officials, a statement from her office said here. The agreement builds on US-Qatari collaboration "to better protect both nations against threats to our common security," said Napolitano said after she signed the document with Minister of State for Interior Affairs Sheikh Abdullah bin Nasser bin Khalifa Al Thani. The document "represents the next step in our partnership as we work together to facilitate legitimate trade and travel between our two nations," she said. According to the US statement, the agreement improves information sharing and strengthens our ability of both countries "to combat transnational crime and other threats" while "facilitating travel and international commerce." Napolitano is scheduled to visit the United Arab Emirates December 5-6, where she will join other US officials to deliver a speech at the Gulf States Global Police Symposium. The United States remains on alert for a possible terror attack, Napolitano told members of Congress in September. The Department of Homeland Security, created in response to the September 11, 2001 terrorist attacks in the United States, includes the US Border Patrol, Immigration and Customs Enforcement (ICE), the Transportation Security Administration (TSA), and the US Coast Guard.