With three planes still grounded, the Israel Air Force will hold a joint experiment next month in the United States in an effort to discover what is causing high levels of carcinogenic material found in the advanced F-16I fighter jet, The Jerusalem Post has learned. The test on the plane will be held in conjunction with the US Air Force, the plane's manufacturer Lockheed Martin and the manufacturer of the engine Pratt & Whitney. The four have already set up a joint team that has met regularly since the crisis erupted. Earlier this month, a team of IAF officers were in the US for meetings with Pratt & Whitney, whose F100-PW-229 is installed in the aircraft. In March, then-OC IAF Maj.-Gen. Elazar Shkedy decided to ground all F-16I training flights after a high level of formaldehyde was found in the cockpit of one of the aircraft. Shkedy suspended training flights after a number of pilots complained of a bad smell coming from the cockpit of one of the planes. The IDF Medical Branch discovered that the smell was caused by a type of formaldehyde known to be carcinogenic in high concentration. Last month, Shkedy decided to approve the planes for flights - after special filters were installed in some of them - but three planes remain grounded with high levels of formaldehyde. By the end of June, filters will have been installed in 23 airplanes. The F-16I - called "Sufa" (Storm) in Israel - is one of the most advanced combat fighter jets in the world, alongside Israel's F-15I. The test, scheduled to be held in July, is expected by the IAF to finally reveal the cause for the high level of formaldehyde discovered on the plane. The IAF suspects that either a part of the engine or the air conditioning system is the cause. Officers have also raised the possibility that the humid weather in Israel was also a factor.