A gunman opened fire on the U.S. Embassy in Lebanon early Wednesday and was shot and captured by Lebanese security forces, adding to mounting tensions in a crisis-hit nation already on edge over months of cross-border strikes between Israel and the armed militant group Hezbollah.

The embassy said that a security guard had been wounded during the attack, without specifying how or whether the injuries were serious.

The motive of the gunman whom the Lebanese Army identified as a Syrian citizen and said was being treated at a hospital, was not immediately clear. Lebanese security forces later detained a second man on suspicion of involvement in the attack after raids in the eastern village of Majdal Anjar, near the border with Syria, according to Lebanon’s state-run news agency.

The news agency did not indicate how the second man might be connected to the attack. Earlier, a Lebanese security official, who spoke on the condition of anonymity because the investigation was continuing, said that initial information suggested the gunman had acted alone.

In the attack on Wednesday, Lebanese security forces and the embassy’s security team responded to “small-arms fire” near the entrance of the fortified compound, which overlooks the Lebanese capital, Beirut, the embassy said in a statement. The Lebanese security official said that the army had later conducted a search of the area around the embassy.

Local news media, citing witnesses, reported that there was a gunfight for almost half an hour before the attacker was shot and detained. Lebanon’s caretaker prime minister, Najib Mikati, said the situation was stable and noted that the American ambassador, Lisa A. Johnson, was not in the country at the time of the shooting.

The embassy was also targeted in September, when a gunman fired on the compound. Nobody was injured in that episode, and a suspect was arrested.

In October, demonstrators protesting after a deadly explosion at Al-Ahli Arab Hospital in Gaza clashed with security forces when they tried to reach the embassy.

The embassy moved from central Beirut to the suburb of Awkar, to the north, after a suicide bombing that killed 63 people in 1983. U.S. officials blamed that attack on Hezbollah, which has been engaged in intense cross-border clashes with Israeli forces since the war in Gaza began last October.

The United States has an outsize presence in Lebanon, where it is the largest international donor to the armed forces, providing more than $3 billion in financial assistance since 2006. The embassy is expected to move soon to an adjacent site, spanning 43 acres, which would make it one of the largest U.S. diplomatic missions in the world.

Hwaida Saad contributed reporting.