WASHINGTON -- The Federal Communications Commission says the nation's four largest wireless carriers have agreed to relay text messages to text-enabled 911 call centers by May 2014. The four carriers -- AT&T, Verizon, Sprint and T-Mobile -- cover 90 percent of the population. However, only a handful of call centers are able to receive texts today, and it's unclear how many will be able to receive them in 2014. Brian Fontes, CEO of the National Emergency Number Association, said the agreement and the associated deadline help call centers plan their technology upgrades and training. NENA organizes state and local agencies that operate such centers. One 911 call center in Iowa started accepting texts in 2009, and trials of 911 texting are going on in Vermont. In other areas, there is no response to texts sent to 911. Under the agreement announced Thursday, texters will get an answer, whether their call center is able to receive texts or not. If it can't, they'll get an automated response telling them to call instead. The FCC said texting to 911 could be useful when a phone call could put the caller in danger. Texting could also aid a person with disabilities who is unable to call. However, the FCC says people should always call 911 during an emergency if they can.