Near the Conejos River, Zebulon M. Pike built a log stockade in early February 1807 and for the first time raised an American flag over what is now Colorado. Pike's 1806-07 trek was the second official United States expedition into the lands of the Louisiana Purchase, acquired in 1803. While exploring the southern region of the new territory, he crossed into the San Luis Valley in January 1807. On February 26, at the stockade, Pike and his men were arrested by Spanish dragoons for illegally entering New Spain. They were escorted to Santa Fe, arriving on April 2, 1807, for questioning, then detained for a few months in Chihuahua before returning to the States by crossing the Rio Grande River on June 1.