premise: Al-Qassam Brigades vowed revenge in a statement.
  A few hours after the incident, a Palestinian was killed and at least two others wounded by Israeli troops' gunfire in the Gaza Strip.
  Monday's violence has cast pall over a visit to the region by US Secretary of State Condoleezza Rice to revive the stalled peace process.
  Israel has stepped up raids and targeted killings of Palestinian militants since a suicide bomb attack killed six Israelis in a seaside city last month.
  The recent worsening of violence has dampened peace hopes ignited by the Israel Gaza withdrawal in September.
   What does the recent worsening of violence refer to?
hypothesis: Israel has stepped up raids