Stocks turned mixed by late morning Tuesday as investor fear over what impact a terrorist attack in Brussels might have on markets faded. 

The S&P 500 was down 0.09%, the Dow Jones Industrial Average declined 0.19%, and the Nasdaq gained 0.09%. 

"It is all to do with the events in Brussels ... but the reaction is muted, which also shows that after all the terrorist attacks we have seen, markets have learned to live with these kind of things," Rabobank eurozone economist Emile Cardon told Reuters.

At least 26 people have been confirmed dead and many more injured after suicide bombers triggered explosions at the airport in Brussels and a metro station, according to reports. The attacks are believed to be in connection with the arrest earlier this week of the main suspect in the Paris terrorist attacks in November. The situation is still developing.

Travel stocks, including airlines and online-booking sites, were hit hardest on the reports. Airlines United Continental (UAL) , Delta Air Lines (DAL) , American Airlines (AAL) and Southwest (LUV) were all lower. Priceline (PCLN) and Expedia (EXPE) were also in decline.

"Investors should ... be braced and positioned for more stock market volatility if in fact the terror attacks continue or larger conflicts begin," Bill Stone, investment strategist at PNC, wrote in a note. "This positioning includes being in an asset allocation that both matches your objectives and ability to withstand price swings."

Safe-haven assets such as gold and government bonds moved higher as traders retreated to less-risky investments. Gold prices rose 0.9% to $1,255.60 an ounce. The yield on the benchmark 10-year Treasury note traded at 1.893% on Tuesday morning compared to 1.921% on Monday.

Crude oil was weaker on Tuesday, alongside equities markets. Commodity traders have contended with worries over U.S. oversupply, while hopes major oil-producing countries can agree on a production freeze has boosted investor appetite. West Texas Intermediate crude oil fell 1% to $41.11 a barrel on Tuesday.

U.S. manufacturing remained subdued as excess inventories stymied production and a stronger greenback hurt international demand. The a flash reading of Markit PMI climbed to 51.4 in March, coming off of a 28-month low in February, though remaining at depressed levels. 

Apple (AAPL) was on watch after federal prosecutors asked a judge to halt a much-anticipated hearing on their efforts to force the tech giant to unlock a phone used by one of the San Bernardino attackers. The FBI may have found another way, and Apple's cooperation may no longer be needed, according to court papers filed late Monday, less than 24 hours before Tuesday's hearing.

Separately, Apple unveiled its latest addition to its smartphone portfolio on Monday afternoon, the smaller, four-inch iPhone SE. 

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Symantec (SYMC) climbed more than 1% after announcing a $1 billion accelerated stock buyback program. The software company has tied the buyback program into its previously-announced $5.5 billion capital return plans, which include a $4 special dividend.

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