document: On Thursday, former FBI director James Comey will testify before Congress, and on the same day the UK's general election takes place.
  The Dow Jones fell 47.81 points, or 0.23%, to 21,136.2, and the wider S&P 500 dropped 6.77 points to 2,429.3.
  The tech-heavy Nasdaq slipped 20.62 points or 0.33% to 6,275.06.
  Retail stocks performed especially poorly.
  Department store Macy's fell more than 8% after executives held an investor meeting and warned the firm's margins would be lower.
  Kohl's slid more than 5%, JC Penney shares fell more than 4%, and US retail giant Walmart dropped 1.2%.
  Even Amazon, the online shopping giant frequently blamed for problems at traditional retailers, closed lower.
  The firm, which has seen its stock soar in recent months, fell 0.8% after it announced it would offer heavily discounted Prime subscription service for those receiving US government aid as it aims to expand the number of people using the service.
summary: Shares on Wall Street closed lower on Tuesday, as retail stocks weighed on markets and political events expected this week kept traders cautious.