The exchanges have to give reasonable time for the press release to be disseminated and wait for a return to an orderly market before they will resume trading.
My understanding is that the exchanges, when they temporarily halt trading in a stock, almost always request other venues to halt trading, but that sometimes the others don't obey - see http://nasdaqtrader.com/trader/news/2003/headtraderalerts/hta2003-164.stm for one example: