Two construction workers are now confirmed dead following a crane collapse Friday in the Yorkville section of the Upper East Side in the second such accident this year.

Police say that Donald Leo, 30, died at the scene. A fellow construction worker said that Leo was planning to marry in a few weeks.

Ramadan Kurtij, 37, of the Bronx, died hours later. A third worker remains in serious condition.

Speaking from the scene Friday morning, Mayor Michael Bloomberg said the crane atop a residential building under construction at 333 East 91st Street appears to have fallen off at or near the 12th floor. The cab of the crane snapped off and the tower appears to have struck the building across the street at 354 East 91st Street, smashing into a corner office, which is believed to have been unoccupied at the time. It then fell to the street below.

"What has happened is unacceptable and intolerable. Having said that we do not at the moment know exactly what happened or why," said Bloomberg. "It would appear that the contractor and the buildings department followed regulations. Whether those were appropriate or not is what we have to see, and whether or not had we had other regulations we could have prevented this, at this point we just don't know."

The crane operator was in the cab at the time of the crash.

Fourteen violations had previously been issued at the site, and an inspector had made a visit last weekend to oversee the crane being raised. Acting Buildings Commissioner Robert LiMandri says an inspector was also there yesterday after complaints of the crane being hoisted over the street. That complaint is still under investigation.

LiMandri says a team of engineers hired by the city in the wake of March's crane collapse will investigate the accident.

"They will also look at the Department of Buildings construction code, they will look at OSHA regulations, they will look at national standards, and they will look at the current regulations scheme that goes towards crane safety and concrete operations," he said. "In addition they will make recommendations as they see fit. And once that goes into effect we will not wait for a report, we will institute that on rolling basis."

Bloomberg says it remains unclear why the top snapped off, but says the city will conduct an investigation. Hundreds of firefighters and rescue workers continue to search through the wreckage, but the mayor says there is no reason to believe anyone is under it.

The building's management said in a statement, "First and foremost, our thoughts and prayers are with the families of those affected by this terrible tragedy. We are currently in the process of gathering all the facts surrounding this unfortunate accident and will provide more details as they become available."

This morning's collapse comes on the heels of a major overhaul in the Department of Buildings after a crane collapse on March 15 killed seven people and damaged several area buildings in Manhattan's Turtle Bay.

But, when asked what he believes is wrong with the DOB, the mayor defended the agency.

“I don't think there's anything wrong with the DOB. DOB didn’t crash; it was the crane that collapsed,” said Bloomberg. “The question that we need to make sure is, Îdo we have the appropriate level of inspection and the appropriate regulations as to what kind of cranes you can use. Keep in mind that construction is a dangerous business and you will always have fatalities.”

The mayor added that while the two crane collapses within a short period of time may appear to be a pattern, but that there is no reason to believe they are connected. Also, the crane that collapsed this morning is a different kind of crane than the one that collapsed in March.

Construction workers on the scene of the collapse are calling the incident horrible.

"I seen the top of the tower, the crane, about two feet in the ground. And a lot of debris. And another gentlemen pinned underneath one of the towers," said construction worker Andy Alvarez. "One of the sections of the crane. All bloody, a lot of debris around him. Horrible, just horrible."

"We didn't want to move anything for fear that something else might trigger something else and pretty much collapse and hurt him," continued Alvarez. "So we just tried to comfort him and clean the litle bit of debris that we could."

"When I heard the sound, I looked up to make sure that nothing collapsed on top of the building and it was folding on me," said construction worker Aaron Broomfield. "So once I seen that and I heard the sound going down, I looked on the outside of the building and saw the crane on the ground."

Another construction worker at the site told NY1 that it is unclear why the crane collapsed.

"The tie-ins are still intact which means it was most likely a problem with the top of the crane. It must have been it couldn't take the pressure," she said. "But there were no loads on it, so we really don't know why it would fall if there's no loads. We didn't have anything. It was swinging around by itself, so we don’t exactly know why it would fall like that."

The worker added that all of the proper inspections had been conducted.

"They was on us like white on rice. We all had to wear harnesses. Most things that they normally wouldn't tell us to do, we definitely had to do that," she said. "We had inspectors every day, building inspectors, inspectors from our union to make sure we was doing everything correctly. We had inspectors from the bottom up. Local of itself had their own inspectors come in. So we really don't know how this can happen 'cause they were on us."

Area residents described a chaotic scene. Matt Bryant, a resident of the building at 354 East 91st Street that was hit by the crane says it hit the 23rd floor — one floor above his apartment. Bryant says he heard a loud noise and felt shaking when the crane collapsed but didn't notice anything out of the ordinary before the collapse.

"The sound was like a thunderclap, like a big crack, like a lightning strike and then it felt like an earthquake was happening in the building," added another resident. "The first thing I did was I immediately went to check on my neighbors — the woman in the corner whose apartment was hit pretty bad by it. It blew out her window. Structurally it took out her terrace and she was in a panic state."

A man from the same building said he had just stepped out when he got a desperate call from his girlfriend that the crane had hit and that water was flooding into the apartment.

"I was sleeping, so it felt like an earthquake and then I woke up and my wall was basically smashed out with glass," added another resident in the building that was hit. "So, it was just glass everywhere. I could hear the people screaming outside. I looked outside there was fire. So I just got my roommate, got my dog and put clothes on and got out of there."

A union representative for the New York City carpenters says one of his members was rushed to the hospital, but is said to be doing well.

"I was in my apartment, I had pictures falling off the wall, I thought it was an earthquake," said the roommate. "Terrifying, absolutely terrifying, and then we were made to evacuate our building. It was awful, it was awful, my next door neighbor's balcony completely wiped out, flooded apartment, it was awful. Awful."

The mayor said the city's first concern is to stabilize the area. Several buildings have been evacuated as a precaution because the vertical part of the crane is still standing and it remains unclear whether it is stable. He added that evacuated residents will be allowed to return at some point later today or tomorrow.

About 160 apartments altogether were evacuated in the following eight buildings: 354 East 91st Street (which is the building that was hit by the crane); and 400, 401, 403, and 404 East 91st Street; and 1750, 1752, 1754 First Avenue.

At 6:30 p.m., all vacate orders were lifted for all buildings save 354 East 91st.

The Red Cross also has several people on the scene and opened a reception center for evacuees at P.S. 198, located at 1700 Third Avenue between 95th and 96th Street.

Many who spoke to NY1 said they were shocked that a crane collapse could happen just two and a half months after the deadly crane accident in March, which led to the arrest of a crane inspector and the resignation of DOB Commissioner Patricia Lancaster.

In April, the DOB launched a $4 million initiative to examine construction sites around the city, sending 20 engineering experts to investigate crane operations, concrete operations and excavations.

The city has added extra inspections at building sites and required that its staff be on hand whenever the towering cranes were raised higher, a process known as a jump. Those procedures are still being revised.

Before today's crash, 13 people had already died in construction-related accidents around the city -- one more than all of 2007.

A number of streets remain closed in the area. Drivers going up First Avenue are being detoured at 86th Street. Crosstown traffic is not allowed to turn on First avenue between 79th and 86th Streets.

The MTA says that due to the crane collapse there are some bus service changes in the area. The M15, M31, M86, and X90 are all on detour.

Many residents in the area have sent photos for NY1 to broadcast on TV. If you'd like to share your photos, can send them to newspix@ny1.com.