As the 30th anniversary of Watergate nears, student investigators at the University of Illinois have concluded that Deep Throat, the White House source who helped journalists unravel the greatest political scandal in American history, was most likely conservative commentator Pat Buchanan. Since 1999, journalism professor William Gaines and his students have set about unmasking the elusive informant. Building on the work of their predecessors—and examining information ranging from thousands of pages of FBI records to interviews with members of the Nixon administration—this year's class unanimously fingered Buchanan, a White House speechwriter during the Watergate era. Gaines, a Pulitzer Prize-winning former...