President Jefferson was also very much interested in the Oregon Country. He had made frequent attempts to have this region explored but all his attempts were unsuccessful. However, after the purchase of Louisiana, he persuaded the federal government to finance, by means of a $2500 appropriation, an official government expedition to make the first overland route to the Pacific Ocean and to explore the region which the United States had recently acquired. He selected Captain Meriwether Lewis of Ivy, Virginia, to head this expedition and Lieutenant William Clark, a very close Virginian friend of Lewis' to accompany him. Their group left St. Louis in the spring of 1804, traveled up the Missouri River, spent a rigid winter in an area now located in North Dakota, continued traveling up the Missouri in the spring of 1805, crossed the Rocky Mountains, and built and paddled canoes until they reached the mouth of the Columbia River in November, 1805. There they built a fort near the present site of Astoria. They remained on the Pacific Coast during the winter and returned to St. Louis in 1806. Lewis and Clark were aided considerably in their travel route directions by an Indian woman guide, Sacajawea. This expedition to the Northwest furnished the best claim of the United States in later ownership disputes with England. Aaron Burr, long the political opponent of Alexander Hamilton, lived in Petersburg. In 1807, Burr was accused of a conspiracy to invade Mexico, to snatch it from Spanish control and to establish an independent Mexican government with himself as the self-appointed ruler. Furthermore, he was accused of having laid plans for setting up a government in the western territory of the United States with the objective of eventually organizing this area into a separate, independent government with himself the self-appointed ruler. Burr was officially tried on a charge of treason at the State Capitol Building in Richmond. Chief Justice John Marshall was the presiding judge. Jefferson, who had disliked Burr for political reasons for a long time and who believed that Burr was guilty of the aforementioned treasonous actions, wanted Burr convicted. Although the trial involved many political entanglements, Burr was finally acquitted.