On 7 January 1943, at the age of 86, Tesla, alone in room 3327 of the New Yorker Hotel, died from coronary thrombosis. His corpse was found by maid Alice Monaghan after she had entered Tesla's room, ignoring the "do not disturb" sign that Tesla placed on his door two days prior to his passing.
The morning after Tesla's death, his nephew, Sava Kosanović, hurried to his uncle's room at the Hotel New Yorker. By the time he arrived, Tesla's corpse had already been removed. Technical papers were missing, as well as a black notebook he knew Tesla kept— a notebook with several hundred pages, some of which were marked "Government." Shortly thereafter Tesla's papers and other property were impounded by the United States' Alien Property Custodianoffice in Tesla's compound at the Manhattan Warehouse, even though he was a naturalized citizen.
A few days after Tesla's death, the information center of the Yugoslav royal government-in-exile released a statement giving a short review of Tesla's achievements and the schedule for his memorial service and funeral. The speech, written by Louis Adamic, was read in a live broadcast on Radio New York by the mayor of New York City, Fiorello La Guardia, on 10 January 1943. The remains of Nikola Tesla were taken to Campbell cemetery. The protocol anticipated the funeral service would be conducted on 12 January in the Cathedral of Saint John the Divine in Manhattan.Bishop William T. Manning delivered the introductory and the last prayer in English. The funeral service was conducted in the name of the Serbian Orthodox Church by priest Dusan Sukletovic, the superior of the Church of St. Sava of the New York parish. The bereaved family members present at the funeral were Sava Kosanovic and Nikola Trbojevic. A state funeral was attended by 2000 people. Tesla's casket was draped with U. S. and Yugoslav flags. The pallbearers were Nobel prize winners. Telegrams of condolence were received from many notables including the First Lady of the United States, Mrs Eleanor Roosevelt and Vice-President Henry A. Wallace. Tesla's body was cremated and his ashes taken to Belgrade, Serbia, then-Yugoslavia in 1957. The urn containing his ashes was placed in the Nikola Tesla Museum in Belgrade. Despite having sold his AC electricity patents, Tesla died with significant debts. Later that year the US Supreme Court upheld Tesla's patent number 645576 in a ruling that served as the basis for patented radio technology in the United States.
Dr. John G. Trump, a well-known electrical engineer serving as a technical aide to the National Defense Research Committee, was the main government official who went over Tesla's secret papers after Tesla's death. Trump was also a professor at M.I.T. and had his feelings hurt by Tesla's 1938 review and critique of M.I.T.'s huge Van de Graaff generator with its two thirty-foot towers and two 15-foot-diameter (4.6 m) balls, mounted on railroad tracks— which Tesla showed could be outperformed in both voltage and current by one of his tiny coils about two feet tall. Trump was asked to participate in the examination of Tesla's papers at the Manhattan Warehouse & Storage Co. Trump reported afterwards that no examination had been made of the vast amount of Tesla's property that had been in the basement of the New Yorker Hotel ten years prior to Tesla's death, or of any of his papers, except those in his immediate possession at the time of his death. After a three-day investigation, Trump concluded in his report that there was nothing that would constitute a hazard in unfriendly hands, stating:
His [Tesla's] thoughts and efforts during at least the past 15 years were primarily of a speculative, philosophical, and somewhat promotional character often concerned with the production and wireless transmission of power; but did not include new, sound, workable principles or methods for realizing such results.
After the FBI was contacted by the War Department, his papers were declared to be top secret. The personal effects were sequestered on the advice of presidential advisers; J. Edgar Hoover declared the case most secret, because of the nature of Tesla's inventions and patents. One document stated that "[he] is reported to have some 80 trunks in different places containing transcripts and plans having to do with his experiments [...]". Altogether, in Tesla's effects, there were the contents of his safe, two truckloads of papers and apparatuses from his hotel, another 75 packing crates and trunks in a storage facility, and another 80 large storage trunks in another storage facility. The Navy and several "federal officials" spent two days microfilming some of the material at the Office of Alien Properties storage facility in 1943, and that was it, until Oct., 1945.
Tesla's family and the Yugoslav embassy struggled with the American authorities to gain these items after his death because of the potential significance of some of his research. Eventually Mr. Kosanović won possession of the materials, which are now housed in the Nikola Tesla Museum.