If this is the case, then the suspects are most likely Sunni Arab guerrillas. However if this is a strategic effort to start isolating the Kurdish north from the Arab central and south, the suspect list is much larger. The usual suspects in that case would be the local Sunni Arabs, Kurdish forces looking to minimize the central government/US ability to intefere with any de jure move to Kurdistan's independence, or the Turkish military seeking to minimize any central support for the Kurds if Turkey decides to invade northern Iraq.
Assuming this was an insurgent attack, it is a part of a growing campaign against bridges. This is at least the fourth successful attack taking down a bridge in the past six weeks. The largest was the Baghdad Tigris River crossing that was completely dropped. An overpass linking two Baghdad Sunni Arab neighborhoods was also recently dropped. A major attack in Mosul destroyed the major link between Mosul and Tal-Afar and the Syrian border.
If the open-source goal of this campaign is to provide further physical system disruption and delegitimization of the government, it is working. From today's attack:
The road leading to Sarha bridge is an important transport link from the capital to the north of the country because it bypasses the insurgency-plagued towns of Samarra, Tikrit, and Baiji in Salaheddin province.
"I left at dawn today with passengers and tried to cross the bridge but instead found hundreds of cars stranded and the bridge badly damaged," said Abbas Hilmi, who runs a taxi between Kirkuk and Baghdad.
"It is a miserable situation for us, it cuts the main road and our main source of income," he added. "We cannot go via Tikrit, that is just too dangerous and under control of armed men."
The Washington Post is reporting the campaign is having an impact in Baghdad:
The tactic has further sealed off neighborhoods, blocked vital transportation links and, in some cases, worsened divisions between Sunnis and Shiites.
Concern about the attacks has led the Iraqi government to prohibit oil tankers and other heavy trucks from crossing all but two of Baghdad's 13 bridges across the Tigris, worsening fuel shortages at a time when drivers must regularly wait hours for gas in lines hundreds of cars long.
"We are really tired of this kind of living," said Amer Abdul Razzaq, 46, the owner of an abandoned hotel and a looted carpet shop, who was visiting a friend near the foot of the Sinak bridge in Baghdad. "We cannot work, we cannot move from one side of the river to the other. These bridges are not military targets; they are affecting the people."