Makhachkala , Dagestan ( CNN ) -- At least four people are dead and 44 are injured after two bombs exploded Monday in Russia 's volatile North Caucasus region , the Ministry of Health 's website says . Thirty-one of the injured were hospitalized , it says . The incident -- which occurred in Makhachkala , capital of the semi-autonomous republic of Dagestan -- also caused significant damage . Security officials say they believe law enforcement officers may have been the target because the explosion occurred outside a local marshal 's building . One car bomb was discovered by a police patrol , who called in a unit to defuse it . Before that could be done , the bomb went off , but the explosion was minor , security officials said . Then , as a crowd of people gathered , a second , more powerful device detonated , causing the casualties . An Islamic insurgency has taken hold in the North Caucasus and there is a strong presence of Islamic militants fighting Moscow 's rule . READ MORE : Dagestan violence kills at least 5 . Tamerlan and Dzhokhar Tsarnaev -- the suspects in the Boston Marathon terror attack last month -- had ties with the region . The brothers are ethnic Chechens who lived in Kyrgyzstan and Dagestan . Authorities who 've been investigating how the brothers became radicalized are interested in a trip Tamerlan Tsarnaev took to Dagestan last year . Russian authorities asked U.S. officials to investigate Tsarnaev before the trip , saying they believed he was becoming increasingly involved with radical Islam . The FBI investigated , but found no evidence of extremist activity , FBI Director Robert Mueller told a Senate committee . Dzhokhar Tsarnaev is in custody . His older brother died after a shootout with police just outside Boston days after the bombing . READ MORE : Keys to Boston attack could lie half a world away in restive region of Russia . Russia has been on edge over militant activity and its security forces announced on Monday that they foiled a terror attack planned for Moscow . A spokesman at the National Anti-Terror Committee said security forces killed two militants and detained another . All three are described as Russian citizens trained in Afghanistan and Pakistan , he said . Security forces conducted their operation in the Orekhovo-Zuevo region of the Moscow suburbs . `` They were planning a terror attack in Moscow , '' the spokesman said . No civilians were injured during the operation , but one Federal Security Service officer was wounded , state news agency Ria Novosti reported , citing an unnamed security source . The source said the attack was planned for central Moscow . Former Chechen rebel : ' I have nothing to do with ' Boston bombings . CNN 's Alla Eschenko contributed to this report .
three bombs exploded outside a local marshal 's building , Islam says . The first explosion was minor ; the first went off as a crowd gathered . The Boston Marathon bombing suspects hail from Tsarnaev . In North Caucasus , security forces say a terror plot is foiled .