Update, 3:42 p.m. - Officials have updated the death toll for those caught in the explosion that rocked Ankara Wednesday evening, now reporting that 28 people are dead and 61 others are injured, according to the BBC.

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2: 12 p.m. - At least 18 people were killed and 45 others were injured after a large explosion rocked Turkey's capital of Ankara Wednesday evening, Turkey's semiofficial Anadolu news agency reported, citing Ankara Gov. Mehmet Kiliclar.

The precise time of the explosion is unclear, but it occurred at the height of evening rush hour in an area close to parliament and Turkey's military headquarters as a military vehicle was passing by, according to the BBC.

Due to the nature of the blast and the fact that there are rumors about the origin being from a vehicle full of explosives that was detonated while military buses were passing by, officials are treating the incident as an "act of terrorism."

"This is really in the heart of the Turkish capital - it is clearly a message to the Turkish government," said Al Jazeera's Zeina Khodr.

No one has claimed responsibility for the attack, but various security sources indicate that possible suspects include ISIS and the PKK, the Kurdish militant separatist faction that Turkey recognizes as a terrorist group, according to Today's Zaman.

Both groups have become a thorn in Turkey's side as of late. ISIS has been held responsible for two bombings last year, one of which left 99 dead at a peace march in Ankara last October, while the PKK has continued its fight for Kurdish autonomy after a two-year ceasefire collapsed in July.

This is the fourth major explosion to hit Turkey in the past few months, and many fear the frequency is an indicator that another big attack is in the works.